A Comprehensive Review of Agricultural Residue-Derived Bioadsorbents for Emerging Contaminant Removal

Abstract

The increasing presence of ECs in aquatic environments has drawn significant attention to the need for innovative, accessible, and sustainable solutions in wastewater treatment. This review provides a comprehensive overview of the use of agricultural residues—often discarded and undervalued—as raw materials for the development of efficient bioadsorbents. Based on a wide range of recent studies, this work presents various types of materials, such as rice husks, sugarcane bagasse, and açaí seeds, that can be transformed through thermal and chemical treatments into advanced bioadsorbents capable of removing pharmaceuticals, pesticides, dyes, and in some cases, even addressing highly persistent pollutants such as PFASs. The main objectives of this review are to (1) assess agricultural-residue-derived bioadsorbents for the removal of ECs; (2) examine physical and chemical modification techniques that enhance adsorption performance; (3) evaluate their scalability and applicability in real-world treatment systems. The review also highlights key adsorption mechanisms—such as π–π interactions, hydrogen bonding, and ion exchange—alongside the influence of parameters like pH and ionic strength. The review also explores the kinetic, isothermal, and thermodynamic aspects of the adsorption processes, highlighting both the efficiency and reusability potential of these materials. This work uniquely integrates microwave-assisted pyrolysis, magnetic functionalization, and hybrid systems, offering a roadmap for sustainable water remediation. Finally, comparative performance analyses, applications using real wastewater, regeneration strategies, and the integration of these bioadsorbents into continuous treatment systems are presented, reinforcing their promising role in advancing sustainable water remediation technologies.

1. Introduction

In recent years, research focused on the removal of ECs (ECs) has gained prominence due to the potential risks these compounds pose to human health and ecosystems on a global scale. These chemical substances are commonly detected at trace concentrations, ranging from nanograms to micrograms per liter, which makes their removal through conventional treatment methods a significant challenge [1,2]. Among ECs, various types of chemical substances are present, including microplastics, dyes, pharmaceuticals, pesticides, personal care products, heavy metals, and industrial additives, among others [3,4,5,6].

Given the diversity and structural complexity of ECs, many of these compounds are non-biodegradable and pseudo-persistent, which significantly hinders their removal through conventional water treatment methods. In addition to their resistance to degradation, these pollutants exhibit environmental behaviors that are difficult to detect, with toxicological effects and removal mechanisms that remain not fully understood after their release into aquatic environments [7,8].

Although several advanced technologies have been investigated to mitigate the presence of these compounds, their application is still mainly limited to laboratory scale due to high operational costs, energy consumption, technical complexity, and the potential formation of toxic by-products. In this context, adsorption emerges as an effective, environmentally safe, and operationally simple alternative, especially when combined with the use of agricultural residues. These residues serve as precursors for adsorbent materials, enabling both contaminant removal and the valorization of agricultural by-products within a sustainable framework [9].

Globally, agricultural activity generates nearly 5 billion tons of waste annually, representing an immense resource that remains largely underutilized [10]. The challenge lies in converting these waste materials into valuable resources through sustainable processes. Recent studies have demonstrated that agro-industrial waste can be effectively upcycled into activated carbons, especially for wastewater treatment applications involving complex pollutants such as dyes [11].

Currently, research efforts are focused on developing innovative materials from agricultural by-products such as rice husks, cassava peels, nut shells, açaí seeds, and other plant-based residues. These adsorptive materials have garnered considerable attention due to their high potential for removing environmental pollutants, particularly when converted into activated carbon or biochar [9]. Carbonization followed by physical or chemical activation significantly enhances surface area, as demonstrated by [12], who used coconut shells to produce activated carbon with specific surface areas reaching up to 2500 m²/g. Lignocellulosic materials, which account for approximately 45% of the raw materials used in activated carbon production, are characterized by high volatile matter and low inorganic contents, properties that promote the development of well-defined porous structures [13]. Therefore, in addition to possessing intrinsic properties favorable for adsorption, these materials contribute to the valorization of agro-industrial waste and help mitigate the environmental impacts associated with improper waste disposal.

2. Agricultural Residues

Agricultural residues are biomass-rich by-products generated throughout the agricultural production chain, including land preparation, harvesting, consumption, and agro-industrial processing. They can be classified in different ways: harvest residues (straw, leaves, cobs), industrial processing residues (sugarcane bagasse, fruit peels), livestock waste, and fruit residues. Among the most used precursors for adsorbent materials are rice husks, sugarcane and grape bagasse, corn stalks, cassava peels, peanut shells, and açaí seeds [14].

It is estimated that thousands of tons of these residues are discarded into the environment every year, raising concerns about their proper disposal and the mitigation of their environmental impacts. Therefore, to reduce this surplus and promote its valorization, the use of agricultural and industrial residues for pollutant removal has gained increasing attention, especially considering the growing intensification of agricultural activities over recent decades [10,15]. In this context, utilizing such materials as adsorbents represents a promising and sustainable alternative, provided that their chemical and structural composition is well understood. Understanding the physicochemical characteristics and properties of these residues is crucial for developing effective and selective adsorbents that can efficiently remove ECs [16].

Their lignocellulosic composition makes these residues valuable precursors for the development of sustainable adsorbents (Figure 1). Cellulose, estimated to account for 35–50% of residue biomass, is the main structural component, providing rigidity and stability to the plant cell wall. It is widely present in materials such as corncobs, pineapple fibers, rice straw, and cotton stalks. Hemicellulose (20–35%), composed of a heterogeneous mix of sugars, acts as a binder between cellulose and lignin, exhibiting an amorphous structure and high susceptibility to chemical degradation [17]. Lignin, on the other hand, contributes to mechanical strength and rigidity, and its complex aromatic structure favors π–π interactions with aromatic compounds—making lignin-rich residues particularly effective in the adsorption of pharmaceuticals and dyes [18]. Therefore, the combined presence of lignin, cellulose, and hemicellulose provides a complex matrix rich in functional groups (–OH, –COOH, –OCH₃), enabling interactions with a wide range of ECs [19].

3. Preparation and Modification of Adsorbents

Agricultural residues have attracted increasing attention as sustainable precursors for bioadsorbent development due to their abundance, renewability, and inherent surface functionality. The performance of these materials in removing contaminants from wastewater is directly linked to their physicochemical properties, which are influenced by the methods used in their preparation and modification. Therefore, this section presents the main approaches employed in the synthesis and functionalization of agricultural-residue-based adsorbents, with a focus on pyrolysis, chemical activation, and surface magnetization.

Before delving into the more advanced modification techniques, it is essential to highlight that raw or minimally treated agricultural residues also exhibit promising adsorption capacities for a wide range of emerging contaminants, including compounds of high environmental persistence. For instance, perfluoroalkyl substances (PFASs), notably perfluorooctanesulfonic acid (PFOS) and perfluorooctanoic acid (PFOA), have been effectively removed using low-temperature pyrolyzed biochars derived from materials such as peach stones, spent coffee grounds, and sawdust. These materials demonstrate adsorption via hydrophobic partitioning and electrostatic interactions, even without full carbonization [20,21]. Similarly, pinewood-derived and reed straw biochars produced under moderate pyrolysis conditions achieved removal efficiencies of up to 169 mg/g and 80%, respectively [22,23]. These findings underscore that non-activated or mildly activated biochars can serve as viable adsorbents in scenarios where simplicity and cost-effectiveness are essential. Thus, the valorization of agricultural residues through low-energy conversion pathways remains a complementary and attractive strategy to the more intensive modification routes explored in the following subsections.

3.1. Pyrolysis Techniques: Conventional Pyrolysis vs. Microwave-Assisted Pyrolysis

Pyrolysis is a thermochemical conversion technique widely applied for the valorization of lignocellulosic biomass, carried out in the absence of oxygen or the presence of a controlled amount of air. During this process, components such as cellulose, hemicellulose, and lignin are thermally decomposed, yielding three primary products: biochar, bio-oil, and non-condensable gases [24,25]. This conversion enables the production of biochar with high energy density and potential applications as an adsorbent, solid fuel, soil conditioner, or precursor for carbon-based materials. Meanwhile, the generated gases (mainly H₂, CO, CO₂, and light hydrocarbons) can be used for energy generation or upgraded into higher-value compounds, provided they exhibit an appropriate H₂:CO ratio [8,9]. Bio-oil, in turn, is a complex and viscous mixture of oxygenated compounds, with high water content and a higher heating value than the original biomass, making it a promising alternative source of liquid energy [26,27].

Pyrolysis can be classified into four main categories based on heating rate, temperature, and residence time: slow, intermediate, fast, and flash pyrolysis [28]. Slow pyrolysis is characterized by low temperatures (300–400 °C), heating rates of 5–7 °C/min, and prolonged residence times, favoring high biochar yields. Intermediate pyrolysis operates between 400 and 500 °C with moderate heating rates (1–70 °C/min) and generates both liquid products and biochar in a balanced ratio. Fast pyrolysis (400–700 °C, heating rate of ~200 °C/min for 2 min) and flash pyrolysis (>500 °C, >1000 °C/min, <2 s) prioritize the production of bio-oil and gases, resulting in lower biochar production. Thus, the higher the heating rate, the greater the formation of liquids and gases; lower rates favor biochar [29].

Microwave-assisted pyrolysis (MAP) emerges as a promising alternative to conventional pyrolysis, employing electromagnetic radiation to heat the biomass rapidly, volumetrically, and selectively [29,30]. In this process, microwaves interact directly with the polar molecules of the biomass, promoting more uniform internal heating, in contrast to conventional heating based on conduction, convection, and radiation [31]. The main advantages of MAP include the higher energy efficiency, precise temperature control, reduced residence time, lower tar formation, and greater development of the porous structure of biochar [32,33]. Additionally, microwave reactors do not require physical contact with heated surfaces, which reduces thermal losses and facilitates the processing of moist or viscous biomass.

However, the performance of MAP is conditioned by the dielectric permittivity of the biomass, which determines its ability to absorb microwave energy. Biomass with low permittivity requires auxiliary absorbers, such as metal oxides, graphite, or silicon carbide, to enable efficient heating [33]. Other technical challenges include the formation of hotspots (overheating zones), greater system complexity, and high infrastructure and operating costs, factors that still limit the large-scale feasibility of MAP [34,35].

Table 1. A comparison between the main operational and structural characteristics of conventional pyrolysis (CP) and microwave-assisted pyrolysis (MAP), highlighting the advantages and limitations of each approach in the conversion of lignocellulosic biomass.

Criterion	Conventional Pyrolysis (CP)	Microwave-Assisted Pyrolysis (MAP)
Heating mode	External, by conduction or radiation	Volumetric and selective (direct interaction with biomass molecules)
Heating rate	Slow	Fast
Heating uniformity	Lower uniformity (temperature gradients)	More uniform heating
Energy efficiency	Lower efficiency	High energy efficiency
Residence time	Long	Short
Process control	Simpler control, less precise	Easier automation and precise control
Interaction with material	Independent of dielectric constant	Dependent on dielectric constant (requires suitable biomass or absorber)
Need for physical contact	Yes (heat transferred via contact with hot surfaces)	No (contactless heating)
Main products	Biochar (high stability), bio-oil, non-condensable gases; lower syngas yield, higher tar formation	Biochar with higher surface area, less tar, H2-rich gas
Industrial application	Broad application and well-established technology	Still limited due to equipment cost and dielectric restrictions

compares the main operational aspects of conventional pyrolysis and MAP.

Recent studies emphasize the advantages of microwave-assisted pyrolysis (MAP) over conventional pyrolysis (CP), particularly in energy efficiency, processing time, and the control of biochar porosity. Durán-Jiménez et al. [36] showed that MAP combined with H₃PO₄ activation produced activated carbons with surface areas over 1100 m²·g⁻¹ in only 2 min, with yields up to 64% and high CO₂ selectivity (4.1 mmol·g⁻¹ at 0 °C). Although both methods resulted in similar surface areas—1260 m²·g⁻¹ for CP and 1145 m²·g⁻¹ for MAP—the MAP sample had superior CO₂ adsorption due to a higher proportion of ultrafine micropores (<0.7 nm). MAP also favored the formation of hierarchical porosity and reduced structural collapse, attributed to rapid energy absorption by the H₃PO₄-impregnated biomass, which increased dielectric permittivity and internal heating efficiency, resulting in improved textural properties and energy savings of 2.4 GJ·ton⁻¹.

Zhou et al. [37] compared conventional and microwave-assisted pyrolysis (MAP) for the treatment of pharmaceutical sludge and found significant differences in product properties. Under the same temperature (600 °C), MAP biochars exhibited higher surface area (146 vs. 92 m²·g⁻¹), attributed to more efficient volumetric heating, which enhances porosity and prevents structural collapse. MAP also resulted in better adsorptive performance, achieving 99% tetracycline (TC) removal, superior to that of conventional biochar. In terms of pyrolytic liquids, MAP produced oils with fewer oxygenated compounds and more aromatic hydrocarbons, and significantly increased combustible gas production (LHV: 21.45 vs. 11.62 MJ·Nm⁻³).

While both CP and MAP are applicable for the thermochemical conversion of lignocellulosic and organic residues, they differ in operational, energetic, and structural characteristics. CP offers simplicity, lower equipment costs, and broad biomass applicability, but relies on slower external heating, has a lower energy efficiency, and may cause structural collapse [37]. MAP, by contrast, provides a higher energy efficiency, shorter processing time, and improved functional selectivity due to its volumetric and dielectric heating, though it requires biomass dielectric compatibility and faces challenges such as hotspot formation and high infrastructure costs [36].

Despite the advances highlighted in recent studies, the application of microwave-assisted pyrolysis (MAP) still faces significant limitations for its large-scale consolidation. Meng et al. [38] demonstrated that increasing microwave power can improve the structure of the produced biochar and its adsorptive capacity for leachate treatment, but it also reduces the yield (from 50% to 35%) and requires the strict control of parameters such as temperature and residence time. Even in systems with more uniform heating, the specific surface area remained relatively low (max. 23 m²/g), indicating the need for additional activation strategies.

Additionally, high infrastructure and operating costs impact MAP’s economic feasibility. Its sustainability relies on the durability and regeneration of biochar across multiple cycles. These challenges highlight the importance of standardizing operational conditions and optimizing energy use to establish MAP as a scalable and environmentally viable solution. Furthermore, biochar properties are also directly influenced by process parameters such as temperature, residence time, atmosphere, biomass composition, and particle size, as discussed in the next section.

3.1.1. Key Influencing Factors

Temperature, residence time, and reaction atmosphere have a decisive influence on the efficiency and selectivity of pyrolysis, directly affecting the yield and quality of pyrolytic products. In conventional pyrolysis, the gradual increase in temperature and longer residence times favor the formation of biochars with greater thermal stability. In microwave-assisted pyrolysis (MAP) systems, heating is volumetric and extremely rapid, promoting selective reactions in a shorter time, but at the risk of hotspot formation and structural collapse. The atmosphere, in turn, defines the oxidizing or reducing nature of the environment, potentially acting as a physical activating agent or negatively influencing the composition of the products.

In addition to these classic parameters, other operational factors also significantly affect process performance, especially in MAP. These include biomass composition, particle size, applied power, and the use of absorbers or catalysts.

Biomass composition is a key factor in the yield and quality of pyrolytic products, especially biochar. Lignocellulosic biomasses with higher lignin contents tend to produce more stable and higher amounts of biochar due to lignin’s greater thermal resistance. In contrast, cellulose and hemicellulose contribute mainly to the volatile fraction, favoring bio-oil formation [35]. Moreover, chemical composition can vary depending on soil type, plant age, and climatic conditions, which underscores the importance of the proper characterization of the feedstock [39].

At higher temperatures (>600 °C), there is an increase in porosity and the formation of gaseous and aromatic compounds, with a lower biochar yield [28]. How this temperature is reached also influences the resulting products: rapid heating to temperatures below 650 °C followed by abrupt cooling favors the formation of liquid products; on the other hand, slow heating to high temperatures maximizes biochar production [35]. In MAP, the applied power determines the heating rate, and excessive temperatures may lead to the secondary pyrolysis of volatiles, reducing bio-oil yield and favoring the formation of non-condensable gases [28,35].

While conventional pyrolysis is less sensitive and predominantly uses N₂, MAP is highly influenced by the reaction medium, since the electromagnetic field interacts directly with the dielectric permittivity of the environment. The presence of CO₂ or steam can act as a mild physical activating agent, modifying the porosity of the biochar and affecting its yield [28,38,40].

Other factors, such as biomass particle size, directly influence the thermal efficiency of microwave-assisted pyrolysis. Smaller particles (<0.25 mm) promote uniform heating and reduce the formation of hotspots—overheated regions capable of causing thermal stress and the structural collapse of the biochar [40,41]. In contrast, larger particles travel through zones with varying electromagnetic field intensities, generating multiple localized heating points [42]. In addition to particle size, the position of the sample within the reactor is also critical, as lower regions of the cavity concentrate higher electric field intensities and promote more efficient heating [41].

The use of absorbers and catalysts is essential in MAP for biomasses with low dielectric constants. Absorbing materials are crucial for such biomasses because they enable effective coupling with the electromagnetic field, promoting the conversion of microwave energy into heat. These absorbers are classified into two main types: (i) dielectric loss absorbers, such as graphite, activated carbon, and silicon carbide, which function through mechanisms like interfacial polarization, dipolar polarization, and electrical conduction; (ii) magnetic loss absorbers, such as metallic ferrites and Fe, Ni, or Co particles, which operate via magnetic resonance, hysteresis, and eddy currents.

In addition, many of these materials also act as catalysts, influencing product selectivity and increasing yield [40]. In conventional pyrolysis, although less common, metallic catalysts like Fe and Ni are also used to modulate secondary reactions and enhance the formation of gases.

Table 2. Comparative influence of operational parameters on conventional pyrolysis (CP) and microwave-assisted pyrolysis (MAP), highlighting general effects on process efficiency, product quality, and biochar performance.

Parameter	Conventional Pyrolysis (CP)	Microwave-Assisted Pyrolysis (MAP)	General Effects
Biomass composition	Less sensitive to dielectric constant; cellulose and lignin yield more stable biochar [35]	Highly dependent on dielectric permittivity; affects energy absorption [43]	Biomass rich in cellulose and lignin results in more aromatic and stable biochar
Particle size	Larger particles (>0.25 mm) require more time for complete carbonization [29]	Smaller particles (<0.25 mm) favor uniform heating and avoid hotspots [40]	Smaller particle size enhances conversion efficiency and increases surface area
Temperature	Gradual increase (up to >600 °C); favors biochar graphitization and carbonization [37]	Rapid and intense; forms micropores and aromatic compounds in minutes (>600 °C) [36]	Higher temperatures reduce biochar yield while increasing porosity and production of gases and bio-oil
Residence time	Long (minutes to hours); progressive carbonization [28]	Short (seconds to minutes); risk of hotspots [44]	Longer residence time improves structural stability but decreases yield
Atmosphere	Less sensitive; commonly uses N2; CO2 can act as mild activating agent [40]	Atmosphere affects microwave absorption by changing the dielectric permittivity; vapors (e.g., steam or CO2) can alter thermal field and promote mild physical activation of biochar [28,38]	Atmospheric conditions influence oxidation, activation mechanisms, and product composition
Pyrolysis power	Indirectly applicable via temperature and residence time [40]	Defines heating intensity; directly linked to hotspot formation [45,46]	Increased power accelerates reaction rate but increases risk of thermal collapse
Absorbers/catalysts	Less common use; catalysts like Fe and Ni applied to control temperature and increase combustible gas production	Essential for low-dielectric-constant biomass; includes dielectric loss absorbers (e.g., silicon carbide, graphite, charcoal) and magnetic loss absorbers (e.g., ferrites and metal particles like Fe, Ni, Co); influences yield, activation, and selectivity [40]	Catalysts and absorbers enhance product yield, activation efficiency, and selectivity

summarizes these comparative effects, allowing an integrated view of the operational differences between the two systems and guiding the selection of the most suitable conditions according to the type of biomass and the desired product.

Thus, it can be observed that the operational parameters of pyrolysis—such as temperature, residence time, atmosphere, biomass composition, applied power, and particle size—act interdependently, directly affecting thermal efficiency, yield, and the quality of the generated products. The choice between these routes should be based on the type of biomass, the desired characteristics of the products, and the technical and economic limitations of each application. Additionally, the post-synthesis modification of adsorbents is a crucial step to enhance their structure and functionality. Techniques such as chemical activation and surface functionalization have been widely employed to increase surface area, introduce active functional groups, and improve performance in adsorption processes. The main approaches applied for this purpose are discussed below.

3.1.2. Energy Efficiency, Economic Feasibility, and Environmental Performance of MAP

Techno-economic assessments (TEAs) and life cycle analyses (LCAs) have recently advanced the understanding of microwave-assisted pyrolysis (MAP) performance at an industrial scale. Zhou et al. [47] reported that 2 kWh/kg of electricity was required to process wood pellets at 800 °C in a continuous MAP reactor, producing 13.7 MJ of syngas, 4.4 MJ of biochar, and 1.0 MJ of tar. This resulted in an overall energy efficiency of 73.3%, which exceeded that of conventional gasification (typically 40 to 65%). Similarly, the MAP of sewage sludge under optimal conditions (500 °C) achieved 87% energy recovery based on the higher heating value (HHV) of the feedstock [48]. However, energy consumption varies significantly depending on reactor design, feedstock characteristics, and operational parameters. Gao et al. [49] reported electricity demands ranging from 0.8 to 2.6 kWh/kg for textile sludge pyrolysis, while Mao et al. [50] observed values from 0.5 to 3.78 kWh/kg for furfural residue, depending on temperature and screw speed. These variations underscore the importance of optimizing process configurations.

Economically, MAP-derived biochars still face limitations. Haeldermans et al. [51] estimated the minimum selling price (MSP) of MAP biochar at EUR 564 to EUR 979 per ton, higher than that of conventional pyrolysis (CP) biochar (EUR 436 to EUR 863 per ton). However, MAP biochars generally exhibit higher surface areas, enhanced porosity, and lower contaminant levels, attributes that may justify premium pricing. Strategies to improve MAP feasibility include the use of microwave absorbers such as SiC and biochar, advanced thermal insulation, energy recovery from pyrolysis gases and char, and the use of low-frequency magnetrons for better scalability.

Additional evidence from Foong et al. [29] confirms that continuous MAP systems, when optimized with energy recovery and dielectric coupling, can achieve a net energy ratio (NER) greater than 0.9, surpassing that of CP. Moreover, the global warming potential (GWP) of MAP was reported to be 62% lower than that of CP under similar operating conditions. These findings highlight MAP’s potential to reduce environmental impacts while enhancing energy performance.

Nonetheless, the industrial-scale deployment of MAP still faces technical and economic challenges, including high capital costs for efficient magnetrons, the need for microwave-compatible biomass, and difficulties in managing heat losses and reactor scale-up. According to Foong et al. [29], integrating heat recovery systems, adopting distributed processing models, and valorizing by-products such as upgraded bio-oil or hydrogen-rich gas are crucial strategies to improve MAP’s techno-economic feasibility.

Therefore, although microwave-assisted pyrolysis offers clear advantages in terms of energy efficiency, process selectivity, and environmental impact, its large-scale implementation depends on reactor optimization, operational stability, and reduced energy and equipment costs. Addressing these factors in future studies will be essential to position MAP as a sustainable and economically viable alternative for biomass valorization.

3.2. Chemical Activation and Functionalization

Chemical activation primarily aims to increase surface area, develop porosity, and introduce active functional groups on the surface of adsorbents, significantly enhancing their ability to capture and remove contaminants from wastewater [52,53]. This process involves treating the precursor biomass with activating agents such as KOH, NaOH, CaCl₂, H₃PO₄, K₂FeO₄, Na₂CO₃, ZnCl₂, or FeCl₃, followed by heating to between 500 and 900 °C under an inert atmosphere. After pyrolysis, washing and drying steps are essential to remove excess activating agents and stabilize the carbonaceous structure [54,55].

Among chemical activating agents, KOH stands out for promoting high surface areas, microporosity, and reactivity, enabling activation at lower temperatures compared to physical methods [56]. In addition, chemical activation can introduce groups such as carboxyls, hydroxyls, and sulfonic groups, which enhance the adsorption of specific contaminants, such as heavy metals or organic dyes [52].

Experimental studies support these effects. According to Shang et al. [57], biochar activated with ZnCl₂ (ZABC-5) showed high efficiency in removing Cr(VI) (98.68%), outperforming material activated with H₃PO₄ (<40%). This performance was attributed to its surface area (34.58 m²/g) and the presence of micro- and mesopores and functional groups (–OH, C=O, C–O) which acted through synergistic mechanisms such as pore filling, electrostatic attraction, ion exchange, and redox reactions. The material also exhibited high stability after four desorption cycles. However, the environmental impact of the process was considered significant (12.70 kg CO₂-eq/kg).

Kaya et al. [58] investigated the use of KOH-activated biochar derived from pine cones for the adsorption of Congo red dye, obtaining a material with an extremely high surface area (1714.5 m²/g) and a removal efficiency of up to 94.62%. In the study, KOH activation played a crucial role in increasing the material’s porosity and promoting the formation of functional groups, including –OH, C=O, and C–H, which facilitated electrostatic interactions, hydrogen bonding, and π–π stacking with dye molecules.

Zhang et al. [59] investigated the production of KOH-activated biochar from wood modified with phenol-formaldehyde resin, obtaining a specific surface area of 2301.61 m²/g and a pore volume of 1.205 cm³/g, attributed to its hierarchical porous structure and the presence of structural defects introduced by the chemical modification of the precursor. The maximum adsorption capacities reached 3472.22 mg/g for Congo red and 1112.35 mg/g for methylene blue. The adsorption mechanisms involved pore filling, electrostatic interactions, hydrogen bonding, and π–π interactions, with the adsorption best described by pseudo-second-order kinetics and the Langmuir isotherm, indicating chemisorption on a homogeneous surface. The prior modification of the biomass contributed to the generation of a greater number of active sites, demonstrating that precursor engineering, combined with the use of KOH, enhances performance.

Hasan Basri et al. [60] and Astuti et al. [61] explored the production of activated carbon from bulrush straw using microwave-assisted pyrolysis (MAP) and ZnCl₂ activation, aiming at the removal of crystal violet (CV) dye. In the first study, Hasan Basri et al. [60] investigated the influence of lignocellulosic composition, and the resulting activated carbon exhibited a surface area of 1141.9 m²/g and a maximum adsorption capacity of 200.7 mg/g, highlighting the role of cellulose and lignin in pore formation and in generating active functional groups such as –OH and C–O. In the second study, Astuti et al. [61] applied multivariate modeling and process optimization, identifying impregnation ratio, pyrolysis time, and ZnCl₂ concentration as key variables that directly influenced adsorption efficiency. Kinetic and isothermal analyses showed the good fit of the pseudo-second-order model and Temkin isotherm, indicating chemisorption with a uniform distribution of binding energies.

In both studies, the use of ZnCl₂ was essential for generating hierarchical porosity and surface functionalization, enhancing the adsorptive capacity of the materials. The application of MAP contributed to rapid and uniform heating, higher yield, and reduced formation of undesired by-products. However, limitations such as high equipment costs, difficulties in heating certain biomasses, and the need for the proper treatment of ZnCl₂ residues were noted. These results indicate that the combination of MAP and chemical activation with ZnCl₂ is effective for producing high-performance activated carbons designed to remove cationic dyes, such as crystal violet.

In addition to the structural improvements promoted by chemical activation, functionalization with groups such as amines (–NH₂), carboxyls (–COOH), and hydroxyls (–OH) enhances both selectivity and adsorption efficiency. These groups enable specific interactions with contaminants through hydrogen bonding, electrostatic forces, and surface complexation. The density of these functional groups depends on the activating agent, pyrolysis conditions, and possible post-synthesis modifications, such as liquid-phase, gas-phase, or ball-milling treatments, as reported by Liu et al. [62].

Despite these advances, gaps remain in the standardization of experimental protocols, which hinder systematic comparisons between different activating agents, pretreatments, and thermal conditions. The variability among studies limits the establishment of universal correlations between structure, functionality, and adsorption performance. Few studies address long-term stability, synergistic effects between functional groups, and behavior in the presence of complex pollutants. Therefore, standardized comparative approaches and the development of multifunctional adsorbents with properties tailored for specific environmental applications are needed [62].

3.3. Surface Engineering and Magnetization

Despite the numerous benefits of adsorbents such as biochar and activated carbon, challenges remain in the efficient recovery of these materials after the adsorption process. To overcome this limitation, magnetic adsorbents have recently been synthesized as an alternative to conventional separation techniques such as centrifugation, sedimentation, and filtration. The incorporation of magnetic properties enables the recovery of the adsorbent using magnetic fields—either by electromagnets or permanent magnets—promoting greater practicality and operational cost savings [63].

Among the most used magnetic materials are iron oxide nanoparticles, such as maghemite (γ-Fe₂O₃) and magnetite (Fe₃O₄), due to their high surface area and ability to interact with a wide variety of contaminants [64]. These oxides exhibit a spinel-type crystalline structure (AB₂O₄), with the distribution of metal cations in tetrahedral and octahedral positions. In the case of magnetite, the inverse structure favors the presence of Fe³⁺ ions in both positions, while Fe²⁺ ions preferentially occupy the octahedral sites [65,66].

In addition to their structural properties, magnetic nanoparticles possess a negative surface charge, which favors the adsorption of cationic pollutants. The combination of these particles with materials containing different ligands increases their affinity for specific compounds, such as heavy metals, dyes, microplastics, and antibiotics. Nanotechnology combined with magnetic separation has proven effective in producing regenerable and selective adsorbents, with applications in the treatment of contaminated water [63,65].

Several studies have explored magnetization strategies applied to different biomasses. Hao et al. [67] produced a porous magnetic biochar from corn straw lignocellulosic residue, modified with Fe₃O₄ via co-precipitation using K₂FeO₄. The resulting material, FeCS800, exhibited high adsorption capacities for Orange G dye (327.87 mg/g) and tetracycline (243.90 mg/g), maintaining stable performance after five regeneration cycles. Its specific surface area and pore volume reached 866.44 m²/g and 0.44 cm³/g, respectively—132 and 22 times greater than the unmodified biochar. The adsorption mechanisms involved electrostatic interactions, π–π stacking, ion exchange, and pore filling, supported by FTIR analysis (–OH, C=O, C–O groups) and magnetic hysteresis (64.08 emu/g), indicating easy magnetic recovery. A high removal efficiency was also observed in natural water, attributed to the formation of abundant oxygen-containing functional groups that form inner-sphere complexes. Additionally, the decreased Fe peak intensities after adsorption suggested that iron oxides acted as extra-active sites, highlighting the importance of magnetic functionalization in adsorption performance.

Peng et al. [68] used corn straw biochar biomodified with phosphate-solubilizing strains and magnetically functionalized with Fe₃O₄ to investigate Pb²⁺ ion adsorption, achieving removal rates above 97%. The specific surface areas of the modified materials ranged from 35.70 to 46.70 m²/g, representing a significant increase compared to the unmodified biochar (BC-0 = 8.56 m²/g). The adsorption mechanisms included ion exchange, surface complexation, and phosphate precipitation, facilitated by surface functionalization with oxygen-containing groups. Microbial modification, by increasing the density of functional groups, played a key role in enhancing adsorption efficiency and metal ion passivation, providing a synergistic approach between chemical and biological functionalization.

Duan et al. [69] evaluated the removal of polystyrene microplastics using magnetic biochar derived from rice straw. The material had a specific surface area of 355.4 m²/g and showed surface-functionality-dependent selectivity. FTIR analysis identified hydroxyl and carboxyl groups as key to adsorption via hydrophobic interactions, π–π stacking, Van der Waals forces, hydrogen bonding, and electrostatic attraction. The biochar demonstrated high regeneration stability and maintained performance across multiple cycles. Microplastics with higher surface functionalization were more efficiently removed, confirming the role of oxygen-containing groups. Even positively charged species (APS) were adsorbed, suggesting additional mechanisms such as π–π conjugation and specific surface interactions. Economic analysis estimated a production cost of USD 0.43/kg and a total cost of USD 2.37/kg, comparable to other biomass-based adsorbents. Magnetic separation and biochar renewability (M-WB) could further reduce wastewater treatment costs, with regeneration estimated at USD 0.40/kg.

Srinadh and Remya [70] produced magnetic biochar from industrial hemp via microwave-assisted pyrolysis (MAP) and FeCl₃ impregnation for Congo red (CR) removal. The material had 10.05 wt% iron, a surface area of 17.64 m²/g, and a 4.84 µm average pore size. CR removal reached 92%, remaining at 87% after three cycles. FTIR confirmed the presence of –OH and C=O groups linked to electrostatic interactions and π–π stacking. The biochar showed low iron leaching and was considered environmentally safe. The authors proposed its reuse as a ceramic additive when mixed with clay, bentonite, or sludge at 1100–1300 °C, enabling the stabilization of oxyanions and reducing environmental impacts.

Despite the advances achieved, significant technical challenges remain. Although Hao et al. [67], Peng et al. [68], Duan et al. [69], and Srinadh and Remya [70] reported excellent adsorption performance, regeneration stability, and cost-effectiveness, most studies were limited to lab-scale tests with synthetic matrices. Applications with real effluents—characterized by complex contaminant mixtures and variable physicochemical conditions—remain scarce. Additionally, the long-term stability of magnetic functional groups and risks of metal ion leaching (e.g., Fe³⁺) pose concerns for safe environmental use. The need for microbial modification [68], high-purity reagents, or MAP [70] can also raise production costs and limit scalability. Furthermore, while adsorption mechanisms like π–π stacking, electrostatic interactions, ion exchange, and complexation are well-documented, studies addressing their synergistic effects in complex systems are still lacking.

To enable the large-scale adoption of magnetic biochars, future research must validate their performance in real wastewater treatment, ensure their long-term functional stability, and develop more cost-effective, sustainable production routes. These steps are critical for establishing magnetic biochar as an efficient, regenerable, and environmentally viable technology.

4. Types of ECs and Adsorption Mechanism

The applicability of agricultural-residue-based bioadsorbents has been widely demonstrated across several categories of ECs, particularly those characterized by persistence, low degradability, and complex molecular structures. Due to their structural diversity and varied physicochemical properties, the effective removal of ECs often requires adsorbent materials with a high surface reactivity, tailored functional groups, and tunable selectivity [2].

Among ECs, pharmaceuticals are particularly concerning due to their continuous discharge from municipal, hospital, and industrial sources. Compounds such as ibuprofen, diclofenac, chloroquine, ciprofloxacin, acetaminophen, and metronidazole are frequently detected in natural water bodies, often persisting through conventional biological treatments. Bioadsorbents derived from lignocellulosic materials such as Parthenium hysterophorus and corncob have demonstrated high efficiency in pharmaceutical removal. The presence of oxygen-containing functional groups and aromatic domains enhances interactions such as hydrogen bonding and π–π stacking. In batch systems, Parthenium-derived biochars exhibited significant sorption capacities for metronidazole and acetaminophen, with synergistic removal effects in multipollutant mixtures. Similarly, corncob biochar functionalized with bimetallic Fe–Cu nanoparticles showed high efficiency in removing ciprofloxacin, not only through adsorption but also via Fenton-like catalytic degradation—an innovative dual-function strategy that integrates adsorption with advanced oxidation [71,72].

Synthetic dyes, extensively used in the textile, paper, and plastic industries, represent another critical class of endocrine-disrupting chemicals (ECs). Their complex aromatic structures confer resistance to light, heat, and microbial degradation, while also exhibiting mutagenic and carcinogenic properties [73]. Agricultural-waste-derived biochars—such as those from peanut shells, pomelo peel, mandarin peels, and corncobs—have been employed for dye removal. Functionalization with metal oxides (e.g., ZnO, TiO₂, CuO, Fe₃O₄) not only improves dye affinity but also imparts photocatalytic properties, enabling hybrid systems capable of simultaneous adsorption and photodegradation. Some of these materials have demonstrated dye removal efficiencies exceeding 95% and outstanding regeneration capacities. Notably, magnetic composite bioadsorbents have been shown to adsorb both dyes (e.g., methylene blue) and pesticides (e.g., acetamiprid) with maximum adsorption capacities exceeding 350 mg/g, highlighting their versatility and applicability in treating complex effluents [74].

Pesticides and herbicides, such as atrazine, glyphosate, 2,4-D, and acetamiprid, are frequently introduced into surface- and groundwater through agricultural runoff. Due to their mobility and toxicity, they pose serious threats to human health and aquatic ecosystems. Biochars and activated carbons produced from sugarcane bagasse, olive pomace, cassava peels, and pistachio shells have been explored as adsorbents for these contaminants. Through chemical activation and magnetic modification, these materials exhibit improved porosity, increased surface functionality, and enhanced binding affinity. In addition to their high adsorption capacities, some systems incorporate catalytic features that promote the degradation of target compounds, further expanding their removal capabilities [74].

Finally, perfluoroalkyl substances (PFASs)—such as perfluorooctanoic acid (PFOA) and perfluorooctanesulfonic acid (PFOS)—represent a particularly challenging class of ECs due to their extreme resistance to thermal, chemical, and biological degradation. While the literature on PFAS removal using agricultural-residue-based adsorbents remains limited, the inherent versatility of biochar surfaces suggests a strong potential for application. Surface functionalization, particularly with nitrogen, oxygen, or iron species, may facilitate interactions with PFASs through electrostatic attraction and hydrophobic partitioning. Moreover, tailoring the point of zero charge (pH_pzc) of the bioadsorbent can improve affinity depending on the contaminant’s ionic state, paving the way for future innovations in PFAS-targeted water treatment [75].

4.1. Adsorption Mechanisms in ECS Removal

The adsorption efficiency of bioadsorbents derived from agricultural residues in the removal of ECs (ECs) is intrinsically governed by the physicochemical interactions between the functional groups of the adsorbent surface and the molecular characteristics of the contaminants. Owing to their lignocellulosic origin and the variety of thermal or chemical modifications they undergo, these bioadsorbents often exhibit a heterogeneous surface with multiple binding sites, allowing the simultaneous occurrence of distinct adsorption mechanisms—including physical adsorption, chemisorption, electrostatic interactions, ion exchange, and even catalytic-assisted transformations. This multifunctionality differentiates agricultural-residue-derived adsorbents from conventional materials such as pristine activated carbon, providing not only versatility but also a tunable surface chemistry adaptable to various classes of pollutants [76].

4.1.1. π–π Interactions and Hydrophobic Forces

π–π interactions represent a fundamental mechanism in the adsorption of ECs onto biochars, acting as a primary link between the adsorbent surfaces and the π-electron systems of the contaminants, such as the aromatic rings present in many pharmaceuticals, pesticides, and dyes. Biochars derived from agricultural residues, rich in lignocellulosic structures, exhibit an abundance of graphitic aromatic rings after pyrolysis, which serve as sites for these interactions. In addition to π–π interactions, hydrophobic forces play a complementary role, facilitating the partitioning of less polar ECs to the hydrophobic surface of biochars. However, it is crucial to acknowledge that the nature and extent of π–π interactions are highly dependent on the specific composition of the precursor biomass. Lignin, a complex and heterogeneous polymer, varies structurally among different plant species, presenting distinct proportions of guaiacyl, syringyl, and p-hydroxyphenyl units. This variability directly influences the electron density, spatial orientation, and degree of aromatic condensation of the resulting biochar. Consequently, biochars derived from different lignin sources (e.g., rice straw versus sugarcane bagasse) may exhibit aromatic sites with distinct characteristics, leading to variations in affinity and selectivity for different ECs via π–π interactions [77,78].

While the presence of π–π interactions is widely established, the direct and universal quantification of these interactions for different lignin sources in biochars remains a complex challenge. Advanced characterization methods, such as solid-state Nuclear Magnetic Resonance (NMR) spectroscopy, X-ray photoelectron spectroscopy (XPS), and X-ray Diffraction (XRD), can provide insights into the graphitic structure and surface composition of biochars, indirectly correlating with π–π interaction capacity. Additionally, computational approaches, such as Density Functional Theory (DFT), can model the interaction energies between the biochar’s π-systems and specific ECs, offering a theoretical quantification at the molecular level. Nevertheless, the practical application and generalization of these methods across a wide range of biochars and ECs are still active areas of research, requiring rigorous experimental validation. A profound understanding of this variability is essential for the rational design of optimized biosorbents for specific EC removal [77,78].

4.1.2. Hydrogen Bonding and Surface Functional Groups

Hydrogen bonding plays a central role, especially when the bioadsorbent retains or acquires surface functionalities such as hydroxyl (–OH), carboxyl (–COOH), and amine (-NH₂) groups through low-temperature pyrolysis, acid/base activation, or heteroatom doping (e.g., N or O). These groups facilitate selective interactions with polar and moderately polar ECs, including pharmaceuticals and pesticides, by forming reversible, site-specific bonds. The strategic introduction of such groups during functionalization processes (e.g., ZnO/N,O-doped biochars) not only enhances adsorption but also allows tailored selectivity toward certain classes of contaminants, which remains underexplored in most large-scale adsorbent applications [72,78].

4.1.3. Electrostatic Interactions and pH Dependence

Electrostatic attraction or repulsion between charged contaminants and the adsorbent surface is strongly influenced by the point of zero charge (pH_pzc) of the material. Below the pH_pzc, the surface is positively charged and favors the adsorption of anionic compounds such as Congo red or PFOS; above it, the surface becomes negatively charged and preferentially adsorbs cationic species like methylene blue or ciprofloxacin. Thus, pH not only modulates surface charge but also alters the ionization state of contaminants, making it a critical parameter in the design of adsorption systems. While this mechanism is often cited, few studies have systematically optimized adsorbent synthesis to tailor the pH_pzc toward specific contaminants—a gap that presents a valuable direction for future innovations [74].

4.1.4. Ion Exchange and Metal–Ligand Complexation

Bioadsorbents naturally enriched with exchangeable cations (e.g., Ca²⁺, K⁺, Mg²⁺), or those modified with metal oxides such as Fe₃O₄ or CuO, can remove ionic pollutants via ion exchange and surface complexation. This is particularly effective for pollutants like glyphosate and heavy-metal-coordinating pharmaceuticals. The formation of inner-sphere complexes between functional groups (–COOH, –OH) and metal-active sites in magnetically modified biochars adds a layer of specificity and strength to adsorption, often resulting in chemisorption-type interactions with high energy bonds [72].

4.1.5. Photocatalytic and Fenton-Assisted Hybrid Mechanisms

One of the most promising innovations involves the integration of bioadsorbents with photocatalytic (e.g., TiO₂, ZnO) or Fenton-like catalytic systems, yielding the synergistic removal of persistent contaminants. In these hybrid materials, adsorption not only concentrates pollutants near catalytic sites but also enhances degradation by facilitating charge transfer, radical generation, and localized oxidation. This dual functionality—adsorption and in situ degradation—enables the partial or complete mineralization of ECs, especially dyes and antibiotics, and represents a cutting-edge frontier in sustainable water treatment technologies [77,79].

4.1.6. Influence of Ionic Strength and Competing Species

In real wastewater matrices, high ionic strength and the presence of natural organic matter (NOM), heavy metals, and co-contaminants can alter adsorption performance. Competing ions may reduce efficiency by blocking active sites or altering the electrical double layer surrounding the adsorbent. However, some bioadsorbents—especially those with meso- and microporous structures—have shown resilience under competitive conditions, a property that could be enhanced through strategic surface engineering. These aspects remain underexplored in long-term or multi-cycle adsorption studies, highlighting the need for pilot-scale investigations of complex matrices [9].

4.2. Adsorption Studies

A comprehensive understanding of the adsorption behavior of ECs (ECs) onto agricultural-residue-based bioadsorbents requires the integration of equilibrium, kinetic, and thermodynamic analyses. These three pillars offer critical insights into the capacity, rate, and feasibility of adsorption under varying environmental and operational conditions. When evaluated in concert, these models not only allow for the optimization of adsorption systems but also elucidate the dominant mechanisms at play—whether physisorption, chemisorption, or complex hybrid interactions—thus enhancing the predictive power and scalability of the process. Kinetic investigations involving ZnO-doped biochar for tetracycline removal and magnetically activated rice husk biochar for 2,4-D herbicide adsorption have demonstrated multi-stage adsorption behavior, characterized by fast initial surface adsorption followed by slower pore diffusion, suggesting that both external and internal mass transfer mechanisms are involved [77,80].

Recent studies on the adsorption of pharmaceuticals and pesticides onto bioadsorbents derived from pistachio shells, wheat straw, and corncobs have consistently shown negative ΔG° and positive ΔH° and ΔS° values, indicating that the processes are spontaneous, endothermic, and accompanied by an increase in entropy. These findings highlight the predominance of specific interactions such as hydrogen bonding and surface complexation and support the use of moderate heating to enhance adsorption performance in practical applications [74].

Together, isothermal, kinetic, and thermodynamic analyses serve as essential tools not only for describing adsorption mechanisms but also for optimizing system design, scaling up applications, and predicting behavior under real-world conditions. However, despite the widespread use of these models, comparative studies using multiple contaminants under dynamic (e.g., continuous-flow) conditions remain scarce and represent a promising avenue for future investigation and innovation in bioadsorbent research. An in-depth analysis of the literature reveals a wide array of agricultural-residue-derived adsorbents that exhibit promising removal efficiencies for structurally diverse ECs. The diversity in precursor materials, activation methods, and functionalization strategies underscores the versatility and tunability of bioadsorbents—yet also highlights the need for comparative evaluations under standardized conditions. For instance, corncob-derived magnetic biochars, modified with Fe–Cu bimetallic nanoparticles, have demonstrated superior performance in removing ciprofloxacin, achieving adsorption capacities exceeding 300 mg/g, and exhibiting additional oxidative degradation through Fenton-like reactions. This dual mechanism significantly enhances pollutant removal, especially in antibiotic-contaminated waters [72].

Similarly, ZnO/N,O-doped biochars synthesized from peanut shells effectively removed both methylene blue and tetracycline, with removal efficiencies exceeding 95%. These systems benefit from a combination of π–π interactions, hydrogen bonding, and photodegradation, illustrating the advantage of adsorption–photocatalysis hybrid systems [77]. Bioadsorbents derived from Parthenium hysterophorus, an invasive weed, offer a low-cost, eco-friendly alternative and have been successfully applied in the adsorption of multiple pharmaceuticals. These materials exhibit high affinity for aromatic compounds and function well in both batch and fixed-bed systems, confirming their scalability and economic potential [81].

In the context of pesticides, magnetically activated rice husk biochar has been employed for the removal of 2,4-D herbicide, where adsorption proceeded via a multistep process, including rapid surface interaction and slower intraparticle diffusion. Despite its lower surface area compared to synthetic adsorbents, its high removal efficiency under optimized pH and ionic strength demonstrates the potential of engineered lignocellulosic adsorbents [80]. Thermodynamic studies across these systems consistently point to spontaneous (ΔG° < 0) and often endothermic (ΔH° > 0) processes, suggesting the prevalence of chemisorption and entropy-driven mechanisms, particularly in systems with strong functional group interactions. This aligns with findings for bioadsorbents derived from wheat straw, pistachio shells, and olive pomace, where performance improved with temperature, indicating the relevance of thermal activation during operational application [74].

To visually synthesize and compare the performance of various agricultural-residue-based bioadsorbents, a radar chart was constructed (Figure 2). This multi-criteria analysis evaluates key parameters—such as adsorption capacity, versatility toward different classes of ECs, surface functionalization potential, regeneration ability, scalability, and cost-effectiveness—based on the recent literature. Such a comparative framework highlights not only the diversity of functional attributes across materials but also the potential trade-offs and priorities in adsorbent design depending on specific water treatment goals.

The radar chart provides a multidimensional assessment of selected agricultural-residue-derived bioadsorbents, evaluated across six core parameters: adsorption capacity, pollutant versatility, surface functionalization, regeneration potential, scalability, and cost-effectiveness. This comparative framework highlights both the functional diversity and trade-offs inherent in bioadsorbent design and application.

Corncob-based Fe–Cu biochar exhibited the most balanced and robust profile. Its high adsorption capacity (q_max = 253.7 mg/g for ciprofloxacin) and broad-spectrum performance are largely due to its dual adsorption–oxidation functionality. The material combines a high surface area with catalytic activity, enabling Fenton-like degradation in the presence of H₂O₂. The process was confirmed to be endothermic (ΔH° = +31.1 kJ/mol) and spontaneous (ΔG° up to −9.5 kJ/mol), indicating chemisorption with increased entropy (ΔS° = +127.5 J/mol·K) [72]. Additionally, its magnetic nature enables easy recovery, reinforcing its suitability for scalable treatment systems.

ZnO/N,O-doped peanut shell biochar showed superior performance in surface functionalization and multi-contaminant removal. With q_max values of 350.8 mg/g for methylene blue and 250.1 mg/g for tetracycline, this material demonstrated fast kinetics (pseudo-second-order) and effective binding via hydrogen bonding and π–π interactions. The system also benefits from photocatalytic activity, enhancing degradation under light exposure. Thermodynamic parameters indicated spontaneous (ΔG° ≈ −4.2 to −7.8 kJ/mol) and endothermic (ΔH° = +25.4 kJ/mol) behavior, aligned with the increased structural disorder during adsorption (ΔS° = +84.1 J/mol·K) [77].

Parthenium-derived biochar, though less functionalized than the engineered composites, achieved significant performance considering its low-cost origin. It showed q_max values of 48.72 mg/g (metronidazole) and 32.10 mg/g (acetaminophen), with pseudo-second-order kinetics and a strong fit to the Langmuir model. Adsorption was confirmed to be endothermic (ΔH° = +23.7 kJ/mol) and spontaneous (ΔG° < 0), with a moderate entropy increase, supporting the feasibility of its application in decentralized or low-resource treatment systems [81].

Magnetic rice husk biochar demonstrated intermediate scores, particularly in regeneration and versatility. It removed 2,4-D herbicide with q_max = 88.3 mg/g, following pseudo-second-order kinetics and a Freundlich isotherm model, suggesting heterogeneous surface interaction. Thermodynamic data (ΔH° = +19.5 kJ/mol, ΔG° ≈ −3.9 to −5.2 kJ/mol) pointed to spontaneous, moderately endothermic adsorption with entropy-driven interactions [80].

The pistachio-shell-based magnetic biochar achieved the highest adsorption capacity, with q_max = 370.4 mg/g for methylene blue and 357.1 mg/g for acetamiprid. The adsorption process, governed by the Langmuir isotherm and pseudo-second-order kinetics, showed strong spontaneous behavior (ΔG° < −9 kJ/mol) and high enthalpy (ΔH° = +29.2 kJ/mol), confirming chemisorption. Its excellent regeneration and magnetic separation properties make it ideal for high-load or complex wastewater systems [74].

In summary, the comparative analysis highlights the multidimensional nature of bioadsorbent performance, where adsorption capacity, selectivity, reusability, and cost must be balanced according to specific treatment objectives. Although each material presents distinct advantages, its effectiveness is also shaped by operational conditions and contaminant complexity. To further understand their applicability, it is essential to examine the performance of case studies involving agricultural-residue-based adsorbents in both controlled and practical scenarios.

5. Application of Adsorbents from Agricultural Residues

Table 3 summarizes biochar adsorbents produced from different agricultural residues, including cereal-derived straws, fruit and nut shells, agro-industrial by-products, and fibrous biomass. These adsorbents have been applied to remove pollutants including pharmaceuticals and personal care products, pesticides, dyes, inorganic pollutants, and emerging micropollutants. Moreover, various surface modification strategies have been investigated to enhance the physicochemical properties and adsorption capacity of biochar adsorbents. These approaches encompass chemical activation, metal and metal oxide functionalization, physical and thermochemical processes, and the development of composite and new materials. Therefore, this section discusses the effect of different operational parameters during the production of biochar adsorbents from agricultural residues, as well as the influence of subsequent surface modification methods on their physicochemical properties.

Liao et al. [82] evaluated the effect of different pyrolysis temperatures (300–600 °C) on the physicochemical properties of corncob and rice husk biochar adsorbents for Pb²⁺ and Cu²⁺ removal. The maximum adsorption capacities were obtained at 500 °C for corncob biochar (61.07 mg/g for Pb²⁺ and 15.29 mg/g for Cu²⁺) and rice husk biochar (60.26 mg/g for Pb²⁺ and 17.35 mg/g for Cu²⁺). The adsorption capacity results for corncob biochar were primarily attributed to the improved pore morphology and increased active sites at 500 °C. These characteristics could have improved mass transfer and the accessibility of Pb²⁺ and Cu²⁺ ions to the active surface. Moreover, the adsorption capacity results for rice husk biochar were mainly related to the increased ash content (28.16–48.31%,) with an enhanced pyrolysis temperature (0–600 °C). The minerals within ash could have provided additional inorganic binding sites for the adsorption of the heavy metals.

Qi et al. [83] studied the effect of microwave-assisted pyrolysis with activated carbon on the physicochemical properties of a wheat straw biochar adsorbent for Pb²⁺, Cd²⁺, and Cu²⁺ removal. The power level of 500 W revealed the highest adsorption capacities, which were 139.44 mg g⁻¹, 52.92 mg g⁻¹, and 31.25 mg g⁻¹ for Pb²⁺, Cd²⁺, and Cu²⁺, respectively. This achievement could be associated with the enhancement in the specific surface area of the wheat straw biochar (from 2.58 to 150.79 m² g⁻¹) with increasing power levels (100–500 W). Moreover, the adsorption mechanism involved complexation with oxygen-based functional groups (–OH and –COOH), ion exchange, and electrostatic interactions between the Pb²⁺, Cd²⁺, and Cu²⁺ and functional groups of the biochar, in addition to the precipitation of metal carbonates (CdCO₃, PbCO₃) on the biochar’s surface.

Nguyen et al. [84] studied the effect of phosphoric acid activation on the physicochemical properties of a sunflower seed husk biochar adsorbent for tetracycline, ciprofloxacin, ibuprofen, and sulfamethoxazole removal. The maximum adsorption capacities for tetracycline, ciprofloxacin, sulfamethoxazole, and ibuprofen using non-activated biochar were 228.3, 80.8, 129.1, and 135.8 mg/g, respectively, whereas for the activated biochar the values increased to 429.3, 361.6, 251.3, and 251.1 mg/g, respectively. These improvements were attributed to the enhanced specific surface area (from 6.2 to 378.8 m² g⁻¹) and total pore volume (from 0.012 to 0.206 cm³ g⁻¹) following activation. Furthermore, phosphoric acid activation introduced additional functional groups on the surface, such as –COOH, –OH, and phosphorus-containing species. For instance, the adsorption mechanisms involved external diffusion, intraparticle diffusion, and chemisorption.

Xiang et al. [85] studied the effect of lignin impregnation on the physicochemical properties of corn straw and wheat stalk biochar adsorbents for tetracycline hydrochloride removal. The maximum adsorption capacities reached 12.61 mg/g for corn straw biochar, 19.08 mg/g for wheat stalk biochar, 15.77 mg/g for lignin-impregnated corn straw biochar, and 32.21 mg/g for lignin-impregnated wheat stalk biochar. These improvements were attributed to the lignin-based impregnation treatment, which increased the specific surface area of the corn straw biochar from 211.55 to 334.19 m²/g and that of the wheat stalk biochar from 123.84 to 478.39 m²/g. Furthermore, the total pore volume was enhanced from 0.1370 to 0.1998 cm³/g for the corn straw biochar and from 0.1349 to 0.3087 cm³/g for the wheat stalk biochar. These changes suggest that lignin-based impregnation treatment fills larger biochar pores and generates smaller ones, resulting in an increased total pore volume and internal surface area. This structural modification resulted in a more mesoporous biochar (mean pore diameter of 2.58 nm), which may have enhanced the accessibility of functional groups and facilitated the diffusion of tetracycline hydrochloride molecules into the biochar pores. Moreover, the adsorption performance of tetracycline hydrochloride on the modified biochar adsorbents can be attributed to physical adsorption mechanisms, including hydrogen bonding, pore filling, π-π interactions, and electrostatic attraction. Furthermore, the Na⁺, K⁺, Mg²⁺, and Al³⁺ ions showed competition with tetracycline hydrochloride for adsorption sites, although Ca²⁺ enhanced the adsorption of tetracycline hydrochloride, resulting in tetracycline–Ca²⁺ complexes.

Abdelfatah et al. [86] studied the effect of nitrogen and sulfur co-doping with a copper zinc ferrite composite on the physicochemical properties of a Beta vulgaris leaf-based biochar adsorbent for reactive black 5 dye removal from simulated and real wastewater effluents. The maximum adsorption capacity of reactive black 5 dye in the simulated effluent was 276.57 mg/g. XPS (X-ray photoelectron spectroscopy) revealed the involvement of sulfur, nitrogen, oxygen, and metal ions within the adsorption process. The interactions included sulfur interactions, with the formation of sulfate (SO₄²⁻) on the biochar surface after adsorption, suggesting the binding of sulfur-containing functional groups from reactive black 5 dye; metal complexation between Cu²⁺, Fe³⁺, and Zn²⁺ from the biochar and the sulfur-containing functional groups in reactive black 5 dye; and coordination complex formation involving oxygen from the reactive black 5 dye and the metals in the biochar. In addition, the adsorption of reactive black 5 dye onto the biochar was favored by hydrogen bonding, caused by the high number of OH groups, and the π-π stacking between the aromatic rings of the biochar and the reactive black 5 dye. Therefore, the synergetic interaction of nitrogen and sulfur co-doping with the copper zinc ferrite composite in the biochar played an important role in the adsorption of reactive black 5 dye. Furthermore, the application of the biochar in real wastewater resulted in a UV-absorbance decrease, in addition to reductions in TDS (430 to 3.5 ppm), turbidity (127 to 63 NTU), and conductivity (860 to 7 µc/cm³). Therefore, the modified biochar was also effective in removing reactive black 5 dye from wastewater effluent, despite the presence of competing ions for binding sites.

Zhang et al. [87] investigated the impact of KHCO₃ activation, Fe₃O₄ magnetization, and La (OH)₃ loading on the physicochemical properties of a corn straw biochar adsorbent for phosphate removal from simulated and real wastewater effluents. The modified biochar showed a maximum adsorption capacity of 116.08 mg/g in the simulated effluent. The mechanisms attributed to phosphate adsorption onto the modified biochar encompassed electrostatic attraction, inner-sphere complexation, ligand exchange, and weak precipitation. The point of zero charge (pH_pzc) was reduced after the adsorption of phosphate, which was attributed to the electrostatic attraction of the negatively charged phosphate and positively charged functional groups from the biochar. XPS indicated a change in the electron density of La after the adsorption of phosphates, suggesting inner-sphere complexation between the La³⁺ and phosphates involving direct coordination and electron transfer. Moreover, the dissolution of surface La (OH)₃ could have provoked the increase in C–OH intensity from 34.71% to 45.42%, which could have been due to the release of OH-groups in the solution. These functional groups may have facilitated ligand exchange or complexation with phosphate, as verified by an increase in OH⁻/O–P intensity from 27.02% to 78.88%. The authors also studied the effect of the modified biochar in the adsorption of phosphates present in river water (Songhua River) and real wastewater (Taiping sewage effluent). The phosphate concentration in the river water was reduced from approximately 190 µg/L to approximately 10 µg/L over 20 min using 1.5 g/L of modified biochar. In addition, the phosphate concentration in the real wastewater was reduced from approximately 1.86 mg/L to approximately 0.16 mg/L over 10 min using 5 g/L of modified biochar. For instance, the modified biochar was effective in removing phosphates even under real operational conditions, such as in river water and real wastewater effluent.

Therefore, future studies should not only simulate the adsorption of target pollutants in the presence of competing compounds but also validate these findings using real wastewater samples. In addition, analytical techniques, such as scanning electron microscopy (SEM), nitrogen adsorption–desorption isotherms (BET), point of zero charge (pH_pzc), Fourier-transform infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS) are important for understanding the morphology of, porosity of, and interactions between the functional groups of pollutants and biochars. Indeed, this data can provide information for the development of new surface modification methods to enhance the adsorption capacity of biochar adsorbents.

Table 3. Summary of biochar adsorbents derived from agricultural residues.

Type of Effluent	Target Adsorbate	Feedstock Type	Surface Modification	Maximum Adsorption Capacity (mg/g)	References
Pharmaceuticals and Personal Care Products	Tetracycline (TC), ciprofloxacin (CIP), ibuprofen (IBP), and sulfamethoxazole (SMX)	Sunflower seed husk	H3PO4	429.3, 361.6, 251.3, and 251.1 for TC, CIP, SMX, and IBP, respectively	[84]
	Tetracycline hydrochloride	Corn straw and wheat stalk	Lignin impregnation	31.48	[85]
	Sulfamethazine (SMT), oxytetracycline hydrochloride (OTC), and amoxicillin (AMX)	Rice straw	KOH	6.47, 87.8, and 7.67 for SMT, OTC and AMX, respectively	[88]
	Ciprofloxacin	Tea waste	MnSO4	121.42	[89]
	Chloroquine	Cassava residue	NaOH and microwave-assisted pyrolysis	39	[71]
	Methyl paraben (MPB), carbamazepine (CZP), ibuprofen (IBP), and triclosan (TCS) from simulated and real wastewater Ciprofloxacin from real wastewater	Empty palm bunch	H2SO4	60.2, 51.7, 38.8, and 35.4 for MPB, CZP, IBP, and TCS, respectively	[87]
		Pomegranate peels	H3PO4	142.86	[90]
Pesticides	Chlorpyrifos	Pomegranate peels	H3PO4	100	[91]
	Imidacloprid	Sugarcane bagasse	KOH with iron/zinc	313	[92]
Dyes	Methylene blue	Wheat straw	Ball-milled biochar with fly ash	854.75	[93]
	Methylene blue	Rice straw	Ammonium phosphate	156.36	[94]
	Methylene blue	Walnut shell	Supercritical CO2 pretreatment and potassium hydroxide activation	80.4	[95]
	Methylene blue	Fronds and leaves from date palm	FeSO4.7H2O	85.1	[96]
	Food Red 17 and Acid Blue 9	Cassava bagasse	NH4Cl	131 and 150 for Food Red 17 and Acid Blue 9, respectively	[97]
	Reactive black 5 dye from simulated and industrial wastewater	Beta vulgaris leaves	Nitrogen and sulfur co-doping with copper zinc ferrite composite	276.57	[86]
Inorganic Pollutants	Fluoride in simulated and glass wastewater	Coconut husks	MnFe2O4 magnetization/Al-La metal–organic framework in chitosan/β–cyclodextrin aerogel	38.59	[98]
	Pb2+ and Cu2+	Rice husk and corn cob	-	61.07 and 17.35 for Pb2+ and Cu2+, respectively	[82]
	Cd2+	Rice straw	Fe-Mn oxides	120.77	[99]
	Pb2+, Cd2+, and Cu2+	Wheat straw	Microwave-assisted pyrolysis with activated carbon	139.44, 52.92, and 31.25 for Pb2+, Cd2+, and Cu2+, respectively	[83]
	Cd2+	Corn stalks	Amino modification	375.58	[100]
	Cd2+	Sesame straw	Alkaline hydrogen peroxide pretreatment	87.13	[101]
	Cd2+ and As3+	Pennisetum sp. straw	Fe-Mn oxides	141.1 and 31.8 for Cd2+ and As3+, respectively	[102]
	Phosphate from simulated, river, and sewage wastewater	Corn straw	KHCO3 activation, Fe3O4 magnetization, and La(OH)3 loading	116.08	[103]
	Phosphate	Orange peels	CaCO3/ZnO	52.96	[104]
	Phosphate	Corncob, sugarcane bagasse, rice straw, and sawdust	Fe2+ and La3+ impregnation	27.49	[105]
	Phosphate	Papaya leaves	Lanthanum-organic framework coating	47.5	[106]
	Tris-(1-chloro-2-propyl) phosphate	Corn straw	-	2.34	[107]
Emerging Micro-Pollutants	Microplastics	Corncob	Fe(NO3)3 and FeSO4	1737	[108]

Table 3. Summary of biochar adsorbents derived from agricultural residues.

Type of Effluent	Target Adsorbate	Feedstock Type	Surface Modification	Maximum Adsorption Capacity (mg/g)	References
Pharmaceuticals and Personal Care Products	Tetracycline (TC), ciprofloxacin (CIP), ibuprofen (IBP), and sulfamethoxazole (SMX)	Sunflower seed husk	H3PO4	429.3, 361.6, 251.3, and 251.1 for TC, CIP, SMX, and IBP, respectively	[84]
	Tetracycline hydrochloride	Corn straw and wheat stalk	Lignin impregnation	31.48	[85]
	Sulfamethazine (SMT), oxytetracycline hydrochloride (OTC), and amoxicillin (AMX)	Rice straw	KOH	6.47, 87.8, and 7.67 for SMT, OTC and AMX, respectively	[88]
	Ciprofloxacin	Tea waste	MnSO4	121.42	[89]
	Chloroquine	Cassava residue	NaOH and microwave-assisted pyrolysis	39	[71]
	Methyl paraben (MPB), carbamazepine (CZP), ibuprofen (IBP), and triclosan (TCS) from simulated and real wastewater Ciprofloxacin from real wastewater	Empty palm bunch	H2SO4	60.2, 51.7, 38.8, and 35.4 for MPB, CZP, IBP, and TCS, respectively	[87]
		Pomegranate peels	H3PO4	142.86	[90]
Pesticides	Chlorpyrifos	Pomegranate peels	H3PO4	100	[91]
	Imidacloprid	Sugarcane bagasse	KOH with iron/zinc	313	[92]
Dyes	Methylene blue	Wheat straw	Ball-milled biochar with fly ash	854.75	[93]
	Methylene blue	Rice straw	Ammonium phosphate	156.36	[94]
	Methylene blue	Walnut shell	Supercritical CO2 pretreatment and potassium hydroxide activation	80.4	[95]
	Methylene blue	Fronds and leaves from date palm	FeSO4.7H2O	85.1	[96]
	Food Red 17 and Acid Blue 9	Cassava bagasse	NH4Cl	131 and 150 for Food Red 17 and Acid Blue 9, respectively	[97]
	Reactive black 5 dye from simulated and industrial wastewater	Beta vulgaris leaves	Nitrogen and sulfur co-doping with copper zinc ferrite composite	276.57	[86]
Inorganic Pollutants	Fluoride in simulated and glass wastewater	Coconut husks	MnFe2O4 magnetization/Al-La metal–organic framework in chitosan/β–cyclodextrin aerogel	38.59	[98]
	Pb2+ and Cu2+	Rice husk and corn cob	-	61.07 and 17.35 for Pb2+ and Cu2+, respectively	[82]
	Cd2+	Rice straw	Fe-Mn oxides	120.77	[99]
	Pb2+, Cd2+, and Cu2+	Wheat straw	Microwave-assisted pyrolysis with activated carbon	139.44, 52.92, and 31.25 for Pb2+, Cd2+, and Cu2+, respectively	[83]
	Cd2+	Corn stalks	Amino modification	375.58	[100]
	Cd2+	Sesame straw	Alkaline hydrogen peroxide pretreatment	87.13	[101]
	Cd2+ and As3+	Pennisetum sp. straw	Fe-Mn oxides	141.1 and 31.8 for Cd2+ and As3+, respectively	[102]
	Phosphate from simulated, river, and sewage wastewater	Corn straw	KHCO3 activation, Fe3O4 magnetization, and La(OH)3 loading	116.08	[103]
	Phosphate	Orange peels	CaCO3/ZnO	52.96	[104]
	Phosphate	Corncob, sugarcane bagasse, rice straw, and sawdust	Fe2+ and La3+ impregnation	27.49	[105]
	Phosphate	Papaya leaves	Lanthanum-organic framework coating	47.5	[106]
	Tris-(1-chloro-2-propyl) phosphate	Corn straw	-	2.34	[107]
Emerging Micro-Pollutants	Microplastics	Corncob	Fe(NO3)3 and FeSO4	1737	[108]

6. Regeneration and Reusability

Although adsorption offers several advantages, one major challenge is disposing of adsorbent materials loaded with pollutants. Improper handling of these materials can lead to secondary pollution, particularly soil and water contamination. Therefore, developing strategies for treating, regenerating, and reusing adsorbents is critical from environmental and economic perspectives, aligning with sustainability and circular economy principles. Reused adsorbents can be employed in subsequent adsorption processes or for alternative applications such as catalysts or biofertilizers. It is important that the adsorbed contaminants can be rapidly released back into solutions, allowing the adsorbent to be reused. The most effective adsorbents offer high potential for regeneration or repeated use.

Various regeneration methods have been studied, but no single technique has proven universally effective for all adsorbents. In some cases, combining two or more regeneration methods has improved performance and could offer a practical solution for restoring spent adsorbents. The choice of method depends on both adsorbent and adsorbate characteristics, including toxicity, combustibility, corrosiveness, radioactivity, and whether the adsorption mechanism is physical (physisorption) or chemical (chemisorption) [109,110,111]. An ideal regeneration technique should fulfill multiple criteria: it should be effective in desorbing pollutants, non-toxic, environmentally sustainable, economically feasible, and operationally simple. Furthermore, the method should ensure that the regenerated material retains its adsorption capacity, especially for applications in water treatment.

Activated carbon typically undergoes 6 to 12 regeneration cycles, depending on the type of adsorbate and the regeneration method employed. In contrast, biosorbents (derived from biological materials) generally exhibit limited regeneration potential. Their mechanical and thermal instability often restricts them to two to six reuse cycles. Despite their low cost and environmental benefits, their lower reusability remains a key limitation in practical applications [112].

6.1. Regeneration of Spent Adsorbents

Regeneration refers to the process of restoring spent adsorbents for reuse in subsequent adsorption cycles. As previously mentioned, it plays a crucial role in determining the practical viability of an adsorbent for large-scale industrial applications, as it directly influences economic feasibility. In this context, several regeneration methods have been proposed in the literature to desorb pollutants from adsorbent materials, including chemical regeneration, thermal regeneration, microwave-assisted regeneration, and techniques based on magnetic separation and recovery.

6.1.1. Chemical Regeneration

Chemical regeneration involves using solvents or reagents to desorb target species from spent adsorbents. Commonly used chemical regenerants include sodium hydroxide, hydrochloric acid, acetone, and ethanol. These agents modify the solution pH to influence the ionization states of the adsorbent surface and the adsorbates, facilitating desorption. The effectiveness of chemical regeneration depends on several factors such as the concentration of the solvent, the solubility of the adsorbates, the surface charge of the adsorbent, and the pH of the solution. Generally, acidic conditions favor the desorption of cationic heavy metals, as these positively charged ions adsorb more effectively in acidic environments. Moreover, pH variations can alter the behavior and binding characteristics of organic pollutants or other organic compounds adsorbed during remediation [109].

Chemical regeneration can be economically advantageous in some cases, particularly when reagents are inexpensive and recovery systems are in place. However, its practical application is often limited by drawbacks such as secondary pollution and potential adsorbent degradation [109]. Therefore, selecting an appropriate eluent is critical for optimizing desorption efficiency. Key considerations include the chemical compatibility of the eluent with both the adsorbent and adsorbate, its ability to disrupt adsorbate–adsorbent interactions, and its pH, which governs ionization and electrostatic forces. Additionally, the potential for complex formation and the presence of organic modifiers can influence desorption performance and must be carefully evaluated during eluent selection [112].

Despite the widespread use of chemical desorbents such as acids, alkalis, chelating agents, and organic solvents, their application presents several challenges. These include secondary pollution, toxicity issues (particularly with solvents like methanol and benzene), the structural degradation of the adsorbent, the potential leaching of nanomaterials, and gradual declines in regeneration efficiency over multiple cycles. Moreover, many chemical regenerants are expensive and require recovery and recycling to maintain economic viability. Nonetheless, chemicals like nitric acid, alkalis, salts, and chelating agents remain prevalent in adsorbent regeneration due to their effectiveness [112,113].

Table 4. Removal efficiency and number of effective regeneration cycles of various agro-industrial residue-derived adsorbents for different adsorbates, with regeneration performed using chemical methods.

Adsorbent	Adsorbate	Solvent	Removal Efficiency (%)	Number of Effective Regeneration Cycles	Reference
Palm kernel shell biochar	Methylene blue	0.1 N HCl and 0.1 M NaOH solutions	97.5	4	[114]
Corncob and luffa sponge activated carbons	Ciprofloxacin	Methanol	80.7% and 78.3%	5	[116]
Activated carbon made from rubber seed and rubber seed shell	Congo red and methylene blue	NaCl (0.01–0.3 mol/L)	97–98%	7	[115]
Sugarcane bagasse	Pb (II) and Ni (II)	0.1 M HNO3, HCl, and NaOH	45–55% for NaOH; 75–79% for HCl; 90–96% for HNO3	-	[117]
Coconut shell and coir	CR (IV)	Solutions of HCl, H2SO4, NaOH (0.2 N), and distilled water	60–100%	4	[118]
Peanut husk with iron oxide biochar	Cr(IV)	0.1 M NaOH	56–96%	4	[119]
Corncob	CR (III) and Cr (IV)	0.1 M NaOH	80–96%	5	[120]
Olive leaves	Cr (IV)	NaOH (0.1 mol/L), NaCl (0.1 mol/L), water	70–100% for NaOH; 60–100% for NaCl; and 30–90% for water	5	[121]
Sweet lime peel biochar	Cr(IV)	1 M HCl, 1 M NaOH, and 0.1 M HCl	80–95% for 1 M HCl; 30–80% for NaOH; 15–80% for 0.1 M HCl	3	[122]
Biochar from rice husk and sewage sludge	Alizarin Red S	Acetone, ethanol, methanol, and subcritical ethanol	60–80% for acetone, ethanol, and methanol; 80–100% for subcritical ethanol	5	[123]
Soybean biochar	Ofloxacin	Ethanol	60–85%	6	[124]
Bermda-grass-derived biochar	Sulfamethoxazole	0.1 M NaOH	50–100%	4	[125]
Seed hulls with iron nanoparticles	Ivermectin	-	83–100%	5	[126]
Cassava-derived activated carbon	Chloroquine	Hydrochloric acid solution (0.2 mol/L)	70–100%	5	[71]
Cassava-derived biochar	Food Red 17 and Acid Blue 9 dyes	NaOH (2 mol/L)	66–100% for Acid Blue 9; 20–100% for Food Red 17	12	[97]

summarizes the main studies on the chemical regeneration of adsorbents derived from agro-industrial residues. It is noted that, generally, higher removal efficiencies are achieved, maintaining removal efficiencies above 70% across multiple cycles. Prabakaran et al. [114] evaluated the reuse of palm kernel shell biochar using 0.1 N HCl and 0.1 M NaOH solutions. They reported that the adsorbent showed good reusability for methylene blue removal, maintaining its effectiveness over four successive adsorption–desorption cycles with only a 2.56% decrease in efficiency. Similarly, activated carbons derived from rubber seed and its shell removed Congo red and methylene blue with high efficiency (97–98%) over seven cycles, indicating strong structural stability and solvent compatibility [115].

Organic solvents also showed promising results. El-Bendary et al. [116] reported desorption efficiencies of 80.7% and 78.3% for ciprofloxacin using methanol on activated carbons derived from corncob and luffa sponge, respectively, with reusability maintained over five cycles. Even more effective was the use of subcritical ethanol in the regeneration of biochars from rice husk and sewage sludge, achieving 80–100% removal efficiency for Alizarin Red S [123]. This suggests that employing solvents under specialized conditions, such as subcritical states, can significantly enhance desorption performance.

The impact of electrolyte concentration on adsorbent reuse was highlighted in studies using NaCl solutions to desorb dyes from rubber-seed-derived activated carbons [115]. Higher salt concentrations led to decreased adsorption efficiencies due to competitive binding and electrostatic repulsion effects between sodium ions and dye molecules on the adsorbent surface. Although a decline was observed after seven cycles, the reduction was relatively small (3% for Congo red and 2% for methylene blue), suggesting reasonable stability under these conditions.

For heavy metal recovery, acid-based desorption methods, particularly nitric acid, have proven most effective. Desorption efficiencies of nearly 90% for lead and over 96% for nickel were reported for sugarcane bagasse adsorbents treated with 0.1 M HNO₃ [117]. This enhanced desorption under acidic conditions is attributed to surface protonation facilitating metal ion release. These results underscore the necessity of coupling high adsorption capacity with efficient desorption and reusability for sustainable metal remediation.

It is also important to highlight the role of adsorbate chemistry and interaction mechanisms in solvent selection. For instance, sweet lime peel biochar showed better regeneration with 1 M HCl (80–95%) compared to NaOH (30–80%) [122]. In contrast, iron oxide-modified biochars and olive leaf biosorbents, where 0.1 M NaOH proved efficient in desorbing Cr(VI), maintained high removal rates over four to five cycles [121].

Regarding reusability, most adsorbents could be effectively reused over at least four cycles while maintaining acceptable removal efficiency. Soybean-derived biochar retained 60–85% of its ofloxacin removal capacity over six ethanol-based regeneration cycles [124], and iron-nanoparticle-coated seed hulls achieved 83–100% ivermectin removal across five cycles [126].

6.1.2. Thermal Regeneration

Thermal regeneration involves heating spent adsorbents to break the interactions between the adsorbent and the adsorbate, restoring the material’s adsorption capacity. This method is typically performed at high temperatures ranging from 300 to 900 °C and often includes oxidative treatment in furnaces, where adsorbed pollutants are decomposed and oxidized. The process generally occurs in two main stages under an inert atmosphere: drying at approximately 105 °C to remove moisture, and pyrolysis and oxidation at elevated temperatures (300–900 °C), which vaporize organic compounds and degrade carbonaceous residues. Despite its effectiveness, thermal regeneration often leads to a 5–15% loss in adsorbent mass per cycle due to structural changes and active site blockage at high temperatures. This material loss limits the number of feasible regeneration cycles and can affect the long-term performance of the adsorbent [127].

Traditional regeneration techniques commonly involve the use of steam or hot inert gases, while non-traditional methods utilize microwave or ultraviolet (UV) radiation. Steam regeneration is particularly effective for activated carbon. The high latent heat of steam leads to a rapid increase in adsorbent temperature, facilitating desorption. Additionally, water molecules can compete with adsorbates for pore space, further enhancing desorption efficiency [109,127].

Pollutant adsorption and desorption efficiency under thermal treatment is closely linked to the activation energy required for these processes. While thermal regeneration offers a robust desorption performance, it is often considered cost-intensive due to its high energy demands and need for specialized equipment and gases (e.g., steam or inert atmospheres). Furthermore, thermal processes can release harmful gases, posing environmental risks such as air pollution [109].

Table 5. Removal efficiency and number of effective regeneration cycles of various agro-industrial residue-derived adsorbents for different adsorbates, with regeneration performed using thermal methods.

Adsorbent	Adsorbate	T (°C)	Time (h)	Gas Flow	Removal Efficiency (%)	Number of Regeneration Cycles	Reference
Durian peel biochar	Ciprofloxacin	400	2	Air	33–72%	3	[129]
Rape straw biochar	Tetracycline	400	1	Air	97%	6	[131]
Bermuda-grass-derived biochar	Sulfamethoxazole	275	3	Air	50–100%	4	[125]
Coconut shell activated carbon	Basic Blue 9 and Acid Blue 93	500	0.5	Nitrogen	42–100%	4	[130]
Magnetic sugarcane bagasse activated carbon	Methylene blue	300	0.5	Air	85–100%	4	[132]
Magnetic biochar from snake fruit peel	Rhodamine B	200	-	Air	99%	5	[133]

summarizes key studies on the thermal regeneration of adsorbents derived from agro-industrial residues. The data indicates that the regeneration temperatures for these materials are generally moderate, typically not exceeding 500 °C, which is considerably lower than the temperatures reported for activated carbon regeneration, sometimes reaching up to 950 °C [128]. Additionally, it is noteworthy that removal efficiency significantly declines after three [129] to four regeneration cycles [125,130], highlighting the limited durability of these biosorbents under repeated thermal treatment. For instance, a sulfamethoxazole-saturated adsorbent regenerated at 300 °C for 3 h achieved nearly 100% efficiency, while lower temperatures between 100 °C and 200 °C were ineffective. However, repeated regeneration cycles at 300 °C led to a decline in performance, with efficiency dropping to 87% after two cycles and 46% after four cycles. This decrease is likely due to structural damage and changes in the surface of the adsorbent. Increased carbon–oxygen vibrations observed after four cycles suggested enhanced surface hydrophilicity, which weakened hydrophobic interactions (the primary adsorption mechanism at pH 3) [125].

Chemical regeneration, although often effective, may result in secondary pollution and adsorbent degradation, whereas thermal regeneration tends to offer greater desorption efficiency at the cost of higher energy consumption [112]. In a comparative study, alkali, thermal, and hydrothermal regeneration methods were employed to restore ciprofloxacin-saturated durian peel biochar [129]. For alkali regeneration, a mixture of 0.1 M NaOH and ethanol (1:1, v/v) was used. Thermal regeneration involved heating the spent adsorbent from room temperature to 400 °C at a rate of 10 °C/min in air, with a 2 h residence time. Hydrothermal regeneration was performed by treating the used adsorbent with 180 mL of deionized water in a 250 mL autoclave at 300 °C for 3 h. After each treatment, the regenerated adsorbent was filtered and rinsed with deionized water. Thermal regeneration showed the highest initial regeneration rate (72%), followed by the alkali (66%) and hydrothermal methods (39%). However, adsorption capacity declined with successive cycles, with regeneration rates dropping to 33%, 23%, and 13%, respectively, after the third cycle. Thermal regeneration was most effective, likely due to its ability to mineralize adsorbed ciprofloxacin and restore active sites. Further characterization of the regenerated adsorbent was performed to investigate adsorbent deactivation mechanisms. The textural analysis revealed that all regeneration methods resulted in a decline in surface area and pore volume over successive cycles. Thermal regeneration retained the highest surface area (666.71 cm²/g after the third cycle), followed by the alkali and hydrothermal methods. The decline was attributed to pore collapse, ash accumulation, and structural degradation, varying by regeneration technique.

6.1.3. Microwave-Assisted Regeneration

Conventional thermal regeneration methods have notable limitations, including being time-consuming and energy-intensive, requiring elevated temperatures, prolonged residence times, and gradual energy transfer. Repeated heating and cooling cycles can lead to the deterioration of the adsorbent’s pore structure, ultimately reducing its adsorption capacity [130]. In contrast, microwave-assisted regeneration presents a more efficient alternative. Unlike conventional heating, in which heat is transferred from the surface inward, often leaving the core cooler, microwave irradiation heats materials volumetrically, generating higher internal temperatures while maintaining relatively lower surface temperatures. This inside-out heating profile accelerates desorption and promotes the more effective breakdown of adsorbed contaminants, making microwave regeneration a promising substitute for traditional thermal methods [127].

Carbon-based adsorbents are particularly well-suited for microwave-assisted regeneration due to their excellent microwave absorption properties, which arise from their interfacial polarization mechanisms. Under microwave irradiation, these materials rapidly achieve high temperatures, thereby increasing their micropore volume and enhancing their adsorption capacity. Additionally, the generation of microplasmas and localized hotspots within the carbon matrix can significantly accelerate the degradation of pollutants trapped in the pores [134]. Therefore, the main advantages of microwave-assisted regeneration are the reduced regeneration time and the tendency to better preserve the original structure of the adsorbent.

Despite these advantages, fewer studies have evaluated microwave-assisted regeneration for adsorbents derived from agro-industrial residues. Wheat straw biochar was regenerated using both a 0.01 mol/L NaOH solution and microwave irradiation [135]. The chemical regeneration with NaOH achieved a regeneration efficiency of 55%. In comparison, the microwave-assisted regeneration demonstrated significantly higher efficiency, reaching up to 98% after treatment at 320 W for 5 min. Over three regeneration cycles at 320 W, the yields were 98.0%, 88.7%, and 85.1%, respectively. The impact was minimal, while repeated cycles slightly reduced microporosity and regeneration efficiency.

Microwave and conventional regeneration methods were compared for recovering dye-saturated activated carbons derived from coconut shells [130]. Microwave regeneration was performed using a single-mode microwave cavity operating at 2.45 GHz under a nitrogen atmosphere, treating 2 g of spent carbon placed in a quartz tube. Conventional thermal regeneration was conducted in a tubular furnace with nitrogen flow, applying a heating rate of 3 °C/min and maintaining the target temperature for 30 min. The best results were obtained with conventional heating at 500 °C for 11 s and microwave treatment at 200 W for 180 s. Although Basic Blue 9 (BB9) dye removal was slightly lower with microwave regeneration, this method was significantly faster than the conventional approach. For the dye Acid Blue 93 (AB93), the adsorption capacity was lower than that of BB9 on fresh carbon, but the microwave-regenerated samples generally showed higher AB93 uptake than BB9. In contrast, conventional regeneration at 500 °C resulted in much lower AB93 adsorption.

6.1.4. Magnetic Separation and Recovery

The development of magnetic adsorbents represents an innovative approach in wastewater treatment, achieved by incorporating magnetic components, such as Fe₃O₄ or CoFe₂O₄, into the adsorbent matrix. These magnetic inclusions allow for the rapid separation of the adsorbent from aqueous solutions using an external magnetic field. Thanks to their high magnetic responsiveness, such materials can achieve effective solid–liquid separation in under 10 s, significantly decreasing the operational time and energy requirements compared to conventional separation techniques, including filtration and centrifugation [136]. Moreover, this functionalization also enhances the adsorption performance of the materials, particularly for heavy metals, dyes, and organic pollutants.

Various authors have shown magnetic adsorbents derived from agro-industrial residues to enable rapid and efficient separation from aqueous solutions using an external magnetic field [126,132,137]. The separation efficiency of the adsorbents from liquid solutions is typically demonstrated through visual documentation (e.g., photographs) and supported by magnetometry analysis. In one study, a magnetic biochar composite demonstrated effective recyclability and ease of separation using an external magnetic field [119]. When regenerated with 0.1 M NaOH, the material retained a high Cr(VI) removal efficiency across multiple cycles. After the first regeneration, the removal efficiency slightly declined from 96% to 90%. By the third cycle, the efficiency remained relatively stable at 72%, but a more pronounced reduction was observed by the fourth cycle, with efficiency falling to 56%. This decline was likely due to partial pore blockage or the loss of active adsorption sites.

Photocatalyst regeneration was performed by magnetically separating biochar powder made from snake fruit peel, followed by washing with ethanol and re-calcination at 200 °C after each cycle [133]. The degradation efficiency remained consistently above 99% through five cycles, indicating excellent material stability and reusability. The effectiveness of recovery was directly related to the magnetic properties of the composite, as confirmed by vibrating sample magnetometry analysis.

Despite the many advantages of magnetic biochars, concerns remain regarding their long-term environmental stability, particularly the potential leaching of Fe³⁺ ions from embedded magnetic particles [138,139]. This phenomenon may occur under acidic or redox-active conditions, posing risks of secondary contamination. Therefore, when considering applications, it is essential to assess the chemical stability of magnetic composites through long-term leaching studies and explore surface modifications that can mitigate iron release while preserving magnetic recovery performance [140,141].

6.2. Reuse and Final Disposal of Spent Adsorbents

Beyond regeneration, managing spent adsorbents requires consideration of both environmental sustainability and economic feasibility. While disposal methods such as incineration and landfilling remain common, reuse strategies aligned with circular economy principles, like soil amendment or catalyst applications, offer promising alternatives but require rigorous toxicity assessments [142]. Overall, various strategies have been employed for managing, disposing of, and reusing spent sorbents, including reuse, incineration, and landfilling.

Several reuse strategies have been explored, including applications in agriculture, capacitor components, and catalyst support materials, while incineration and landfilling remain common and established disposal methods [137]. For instance, in the study by Priya et al. [143], the authors proposed that their adsorbent could be reused as a slow-release fertilizer. The adsorbent was cellulose derived from rice straw for phosphate removal from secondary-treated wastewater. According to the authors, the material was biodegradable and was used as fertilizer, enhancing plant growth, similarly to the growth improvement promoted by commercial fertilizer. Other authors have produced adsorbents from agro-industrial residues and evaluated their reuse for latent fingerprint detection [114,144,145]. In the study by Prabakaran et al. [114], spent biochar powder derived from palm kernel shells was utilized for latent fingerprint detection. The powder was applied to fingerprints previously deposited on glass slides, and excess material was carefully removed using a hairbrush. This process revealed well-defined ridge patterns under natural light. The fingerprints were subsequently documented using a smartphone camera, demonstrating the potential of this spent adsorbent for low-cost and effective forensic applications. In the study by Adeiga et al. [109], a spent macadamia nutshell composite adsorbent was utilized for the photocatalytic degradation of ciprofloxacin, resulting in 86% of the pharmaceutical being efficiently degraded within 80 min.

Once the adsorbent can no longer be regenerated or reused, its safe disposal becomes critical. Incineration and landfilling remain the most common disposal methods, although each has significant limitations [137]. Incineration, often regarded as a waste-to-energy strategy, allows spent adsorbents to serve as alternative fuels to coal, offering reduced corrosiveness and lower toxic gas emissions. However, this method may require pretreatment and does not eliminate the risk of toxic emissions. Landfilling also requires the prior assessment of adsorbate concentration to determine its suitability for disposal, and often mandates pretreatment steps for hazardous materials [146]. Advanced stabilization and solidification techniques can enhance the safety of landfilled materials [147]. For example, zirconium-based compounds have been employed to stabilize arsenic-laden adsorbents [148]. Cementitious binders are also frequently employed to reduce the mobility of contaminants [149]. Other disposal methods include physical separation techniques such as washing, leaching, and chlorination, as well as thermal processes like sintering (including plasma, microwave, and flash sintering), melting, and vitrification. While these methods can produce stable waste forms that comply with regulatory standards, they are often limited by low waste-loading capacities, high energy demands, long processing times, and risks such as hydrogen generation. In this context, cold sintering has emerged as a promising low-temperature alternative. This technique densifies inorganic powders using a transient liquid phase under moderate pressure and temperatures below 300 °C. Cold sintering has shown success in processing a wide range of inorganic materials and has recently been extended to the immobilization of radioactive isotopes such as Cs, I, Ni, and Co in various host matrices [150].

7. Integration of Bioadsorbents into Water Treatment Systems

Although several studies have demonstrated the efficiency of bioadsorbents derived from agricultural waste under laboratory conditions, their practical application depends on additional factors, such as integration into real treatment systems, scalability, operational stability, economic viability and regulatory compliance, as illustrated in Figure 3. The practical incorporation of bioadsorbents into operational water treatment units requires evaluating real water systems on a laboratory scale, expanding their use to large-scale operations. In addition, the verification of operational stability is crucial in order to ensure stability under various operating conditions. Economic and regulatory perspectives verify the financial viability of applications by legal standards and guidelines [150,151,152,153,154].

Although bioadsorbents show promising performances, the transition from laboratory-scale to industrial applications still represents a major challenge. Variability in raw material availability, irregularity in the physicochemical characteristics of adsorbents, and the absence of standardized production protocols are the factors that limit scalability. Furthermore, real wastewater matrices present complexity due to the competition between ions, variations in pH, and variable contaminant loads, aspects that are often not considered in laboratory analyses [155,156,157,158]. Future projects should focus on continuous-flow systems, performing technical–economic analyses (TEAs) and life cycle assessments (LCAs) to determine cost-effectiveness and environmental impact, ensuring sustainability and economic competitiveness [159,160]. In addition, it is necessary to evaluate the performance and stability of long-term regeneration under operational conditions, as well as establish quality control standards for bioadsorbent production, considering aspects such as surface area and porosity [155,156,158].

Ferreira Junior et al. [161] carried out the prototyping and economic evaluation of the use of an anionic bioadsorbent from sugarcane bagasse in the treatment of drinking water contaminated by arsenic. Technological innovation was technically feasible, with an 80-fold increase in synthesis scale and level 4 technological readiness. Regarding the financial analysis of the proposed business model (180,000 units in 5 years in a semi-industrial process, 4% of the Latin American market, and a selling price of USD 6.95 per unit), the authors found that the cash flow showed a return on investment above 20% per year, which is attractive for investments.

Another limiting factor for the use of biosorbents is regulatory barriers, due to the lack of official classification and standardization for these materials. In most countries, environmental agencies do not yet recognize biosorbents as approved materials for tertiary or advanced treatment stages, especially when derived from residual biomass. In the European Union and the United States, materials used in water treatment must comply with strict guidelines, such as the EU Drinking Water Directive (2020/2184) and the EPA’s National Primary Drinking Water Regulations [162,163]. In Brazil and Latin America, similar concerns are governed by national frameworks, such as CONAMA Resolution 430/2011 [164], which focuses on the quality of treated effluents but does not yet specify standards for biosorbents.

7.1. Fixed-Bed and Continuous-Flow Systems

The adsorption operation in fixed-bed columns is simple, provides high yields, and can be easily expanded through a laboratory procedure. Under continuous-flow conditions, liquid-phase adsorption analysis can be performed using breakthrough curves in a column system. The consistency between the results obtained in batch and fixed-bed systems suggests that batch data can serve as a basis for designing and optimizing continuous water treatment systems. With the adequate optimization of operational parameters, it is possible to transpose batch data to applications in continuous systems, promoting sustainable and efficient solutions for the treatment of contaminated water [81,155]. Some studies have used sugarcane bagasse [155], palm oil residues [165], Peltophorum pterocarpum tree pods [80], and industrial lignin obtained from pulping and paper manufacturing processes [166] in fixed beds as bioadsorbents.

Although laboratory column adsorption research has shown efficient results, it is essential to conduct studies on larger-scale columns to understand how the parameters of the fixed bed (such as adsorption capacity, breakthrough time, saturation time, and the size of the mass transfer zone, among others) are impacted by increasing the scale. This will allow us to understand the true potential of this technology and the possibility of applying it on an industrial scale [155]. Ramola et al. [167] evaluated the effectiveness of sugarcane bagasse and pruned bamboo biochars modified with Fe in the adsorption of Pb²⁺ and Cu²⁺ ions in fixed-bed systems, aiming at practical applications in the treatment of contaminated waters. The modified biochars demonstrated high efficiency in removing Pb²⁺ and Cu²⁺, with adsorption capacities superior to those of unmodified biochars. The sugarcane bagasse biochar modified with Fe was more effective in removing Cu²⁺. Comparing the results with the batch performance, the iron-modified biochars also maintained considerable efficiency in the fixed bed, although with a slight reduction in capacity due to hydrodynamic limitations.

7.2. Hybrid and Integrated Technologies

Combining adsorption with additional technologies can enhance treatment efficiency, especially in complex wastewater systems. This is because each pollutant removal process is distinct, unique, and best suited to the removal of a particular type of pollutant. Thus, by combining processes, different mechanisms work together to improve the removal of a broader range of pollutants. Pollutants that are not effectively removed by one process are efficiently removed by another process [168]. Table 6 presents recent studies of photocatalytic systems, Fenton-type processes, and membrane filtration associated with adsorption which have been effective in removing a variety of pollutants, such as dyes, antibiotics, pesticides, heavy metals, and nutrients.

The combination of adsorption and photocatalysis balances the advantages and disadvantages of both techniques, allowing the more efficient degradation and removal of contaminants from water. The isolated use of each process can result in high costs for adsorbent regeneration and the generation of secondary pollution. In synchronized applications, adsorption can improve the contact between photocatalysts and pollutants, while adsorbents with good charge transfer properties can accelerate the separation and transport of photoinduced electron–hole pairs, increasing photocatalytic efficiency. In this context, the integration of adsorption and photocatalysis has emerged as a promising hybrid approach in which the adsorbent acts as a support for the photocatalyst, promoting the concentration of contaminants on its surface and facilitating their exposure to the photocatalytic effect [79,169].

Metal oxide semiconductors are used due to their non-toxicity, self-cleaning, harmlessness, and high photocatalytic activity [79,169]. In the photocatalytic–adsorbent systems in Table 6, the high removal efficiencies for different classes of dyes and antibiotics stand out, with contact times ranging from 30 to 140 min. These systems combine the efficient adsorption of pollutants with photochemical degradation, promoting not only physical removal but also the partial or total mineralization of contaminants. The incorporation of semiconductors such as ZnO, TiO₂, and CuO into the modified biochars significantly improves the separation of photoinduced charges as well as the catalytic activity.

Table 6. Technologies for the use of bioadsorbents with integrative technologies.

Technologies	Agricultural Residues	Composites	Degradation	Main Results	Reference
Photocatalytic–adsorptive systems	Peanut shell	ZnO/N,O-containing biochar	Dye (methylene blue) and antibiotics (tetracycline hydrochloride)	Removal efficiency of 96.0 and 97.1% and contact time of 70 and 140 min for methylene blue and tetracycline hydrochloride, respectively	[77]
	Fallen sycamore leaves	Lamellar sycamore leaf TiO2/biochar	Dye (methyl orange) and antibiotics (ciprofloxacin)	Removal efficiency of 95.7 and 80.8% and degradation of 74.2 and 50.2% for methyl orange and Ciprofloxacin, respectively	[79]
	Pomelo peel	Sn quantum dot-loaded N- and O-containing biochar	Dye (methylene blue, malachite green, and rhodamine B)	Removal efficiency of >90% in 60 min after 5 cycles	[78]
	Corn plants	Corn plant biochar and manganese (Mn)-composited copper oxide (CuO)	Dye (Congo red and Eriochrome Black T)	Removal efficiency of 98 and 95%, and degradation of 92 and 88% for Congo red and Eriochrome Black T, respectively	[170]
	Mandarin peels	TiO2/mandarin waste peels (cellulose source)	Dye (methyl orange)	Removal efficiency of 98.9%, contact time of 30 min, and adsorption capacity of 104.2 mg/g	[169]
Fenton-like processes	Pistachio shell	Biochar-based Fe3O4 nanoparticles and ascorbic acid	Dye (methylene blue) and pesticides (acetamiprid)	Adsorption capacity of 370.4 and 357.1 mg/g for methylene blue and acetamiprid, respectively	[74]
	Corncob	Biochar-based magnetic Fe–Cu bimetallic	Antibiotic (ciprofloxacin)	Removal efficiency of 93.6%, contact time of 360 min, and degradation of 66%	[72]
	Rubber tree bark and coconut shell	Biochar with FeCl3 and H2O2 solutions	Dye (methylene blue) and hexavalent chromium (Cr (VI))	Adsorption capacity of 335.6 and 258.1 mg/g for methylene blue and chromium, respectively	[171]
	Wheat straw	Iron tailings and wheat straw blends	Dye (methylene blue)	Removal efficiency of 84.0%	[172]
	Laurel leaves and watermelon peels	Hydrochars	Dye (anionic—reactive red 180; cationic—basic red 18)	Removal efficiency of 99.8 and 98.8% for reactive red 180 and basic red 18, respectively	[173]
Membrane filtration pretreated with adsorption	Corn stover	Biochar and ceramic membrane filtration	Nitrogen, phosphorus, and organic matter	Removal efficiency of 91.42%, 91.49%, 89.54%, and 76.34% for total nitrogen, total ammonia nitrogen, total phosphorus, and soluble chemical oxygen demand, respectively	[174]

Table 6. Technologies for the use of bioadsorbents with integrative technologies.

Technologies	Agricultural Residues	Composites	Degradation	Main Results	Reference
Photocatalytic–adsorptive systems	Peanut shell	ZnO/N,O-containing biochar	Dye (methylene blue) and antibiotics (tetracycline hydrochloride)	Removal efficiency of 96.0 and 97.1% and contact time of 70 and 140 min for methylene blue and tetracycline hydrochloride, respectively	[77]
	Fallen sycamore leaves	Lamellar sycamore leaf TiO2/biochar	Dye (methyl orange) and antibiotics (ciprofloxacin)	Removal efficiency of 95.7 and 80.8% and degradation of 74.2 and 50.2% for methyl orange and Ciprofloxacin, respectively	[79]
	Pomelo peel	Sn quantum dot-loaded N- and O-containing biochar	Dye (methylene blue, malachite green, and rhodamine B)	Removal efficiency of >90% in 60 min after 5 cycles	[78]
	Corn plants	Corn plant biochar and manganese (Mn)-composited copper oxide (CuO)	Dye (Congo red and Eriochrome Black T)	Removal efficiency of 98 and 95%, and degradation of 92 and 88% for Congo red and Eriochrome Black T, respectively	[170]
	Mandarin peels	TiO2/mandarin waste peels (cellulose source)	Dye (methyl orange)	Removal efficiency of 98.9%, contact time of 30 min, and adsorption capacity of 104.2 mg/g	[169]
Fenton-like processes	Pistachio shell	Biochar-based Fe3O4 nanoparticles and ascorbic acid	Dye (methylene blue) and pesticides (acetamiprid)	Adsorption capacity of 370.4 and 357.1 mg/g for methylene blue and acetamiprid, respectively	[74]
	Corncob	Biochar-based magnetic Fe–Cu bimetallic	Antibiotic (ciprofloxacin)	Removal efficiency of 93.6%, contact time of 360 min, and degradation of 66%	[72]
	Rubber tree bark and coconut shell	Biochar with FeCl3 and H2O2 solutions	Dye (methylene blue) and hexavalent chromium (Cr (VI))	Adsorption capacity of 335.6 and 258.1 mg/g for methylene blue and chromium, respectively	[171]
	Wheat straw	Iron tailings and wheat straw blends	Dye (methylene blue)	Removal efficiency of 84.0%	[172]
	Laurel leaves and watermelon peels	Hydrochars	Dye (anionic—reactive red 180; cationic—basic red 18)	Removal efficiency of 99.8 and 98.8% for reactive red 180 and basic red 18, respectively	[173]
Membrane filtration pretreated with adsorption	Corn stover	Biochar and ceramic membrane filtration	Nitrogen, phosphorus, and organic matter	Removal efficiency of 91.42%, 91.49%, 89.54%, and 76.34% for total nitrogen, total ammonia nitrogen, total phosphorus, and soluble chemical oxygen demand, respectively	[174]

The use of Fenton-type catalysts is crucial in wastewater treatment, as current catalysts lack antiparasitic properties and exhibit reduced activity at high generation rates. This is due to the intrinsic contradiction between the slow sorption rate and the rapid and inefficient utilization of free radicals [175]. Hybrid systems that combine adsorption and Fenton or Fenton-like processes, using agricultural residues as precursors for bioadsorbents, increase their effectiveness in eliminating ECs. In Fenton/biochar processes, the efficiency and removal characteristics are improved due to the movement of electrons between the biochar and the active sites of the iron or redox cycles. Furthermore, the existence of unequal electrons in persistent free radicals in biochar can accelerate the exchange of electrons and intensify the degradation rate of the contaminated pollutant [172].

The choice of residue and appropriate modification are crucial to optimizing system efficiency. The effects of several operational factors, such as the solution pH, catalyst quantity, hydrogen peroxide (H₂O₂) concentration, initial concentration of the emerging contaminant, and structure of the biosorbent, must be considered when combining these systems [173]. The combination of adsorption and Fenton processes focuses on dye removal and the use of agro-industrial shells (Table 6). Fenton-like processes also show promising results, with high adsorption capacities for organic and inorganic contaminants. The presence of metallic nanoparticles (such as Fe₃O₄ or Fe–Cu) activated by reducing agents or peroxides enhances the generation of hydroxyl radicals, which are highly reactive in the degradation of pollutants. These systems benefit from the porous and functionalized nature of biochars derived from waste such as pistachio shells, wheat straw, and corncobs.

Organic micropollutants (MPOs) are efficiently removed by membrane filtration. Although this technique has advantages such as not needing chemicals, high efficiency, and selective rejection, fouling is a significant limitation that can be reduced through pretreatment using adsorption [176]. As shown in Table 6, studies using bioadsorbents as a pretreatment for filtration are still scarce. Lin et al. [174] developed a high-performance ammonia removal, biochar adsorption, and ceramic membrane filtration process for the treatment of kitchen waste biogas sludge. The authors found that integrating bioadsorbent with filtration increased phosphorus removal and reduced membrane contamination. In addition, the by-products of the process were reusable (ammonium sulfate, biochar, and concentrated sludge), which could offset costs through agricultural applications such as fertilizers and soil amendments.

Wang et al. [177] performed a synergistic removal of ultrafine coal particles and Ca²⁺/Mg²⁺ from mine wastewater by two-stage adsorption before membrane filtration. The authors found that the first-stage adsorption was driven by ultrafine coal particles within the bulk solution, and the cake layer formed on the membrane surface initiated the second-stage adsorption. Thus, suspended solid removals of greater than 99.9% and Ca²⁺/Mg²⁺ ratios of more than 20% were achieved.

As evidenced, technologies used in integrated ways improve the efficiency of contaminant removal. In this sense, Mu et al. [178] studied a process combining Fenton oxidation and adsorption followed by filtration through a ceramic membrane. The authors improved the treatment efficiency and operational cycle of refinery effluents. Activated carbon was used in the adsorption process, demonstrating the potential for future studies to assess the efficiency of alternative bioadsorbents.

Although the hybrid systems listed in Table 6 have demonstrated high removal efficiency, it is important to highlight that most of these results were obtained using synthetic or simplified model solutions. Real wastewater matrices typically present more complex compositions, including competing ions, organic matter, and fluctuating pH, which can significantly influence adsorption and catalytic behavior. Therefore, further validation in real wastewater scenarios is crucial in order to confirm the practical applicability and robustness of these hybrid systems.

8. Conclusions

This work presented the wide range of materials that can be developed from agricultural residues to produce advanced bioadsorbents aimed at removing ECs from aqueous media. Biochars obtained through thermal treatments, such as conventional and microwave-assisted pyrolysis, demonstrated increased porosity, high surface area, and partial graphitization—features that enhance π–π interactions with aromatic contaminants like diclofenac and chloroquine. These effects were even more pronounced in systems using biochar derived from Parthenium hysterophorus or corncob functionalized with metallic nanoparticles.

Chemical modifications, including activation with ZnCl₂, KOH, or H₃PO₄ and functionalization with metal oxides (ZnO, Fe₃O₄, CuO), further improved the adsorbents by increasing their surface acidic group density and introducing catalytic properties. These hybrid materials not only enhanced adsorption performance but also enabled the partial degradation of contaminants, as seen in the Fe–Cu-modified biochars used for ciprofloxacin removal via adsorption and Fenton-like reactions.

The main interaction mechanisms between adsorbents and adsorbates included hydrogen bonding (favored by –OH and –COOH groups), ion exchange (necessary for compounds like 2,4-D and glyphosate), and pH-dependent electrostatic forces relative to the materials’ point of zero charge. Surface-charge-tuned materials demonstrated selective affinity toward different ionic species such as PFOS and methylene blue.

High adsorption capacities (>300 mg/g) were reported across a broad spectrum of contaminant classes. Several materials also exhibited regeneration potential and have shown promise in preliminary continuous system applications. However, it is crucial to recognize that many findings are still limited to laboratory-scale studies. While fixed-bed columns and real wastewater trials have provided encouraging results, challenges such as adsorbent fouling, regeneration efficiency, and long-term stability remain underexplored at the pilot and industrial scales.

Thus, adsorbents derived from modified agricultural residues represent a promising and environmentally sound alternative to conventional materials. Beyond their high performance and low cost, these materials contribute to circular economy practices and sustainable water treatment. Further efforts in upscaling, durability assessment, and integration into real treatment systems are essential to ensuring their practical viability.